diff --git a/Microsoft.Azure.Cosmos/src/ConnectionPolicy.cs b/Microsoft.Azure.Cosmos/src/ConnectionPolicy.cs
index ef82aca225..717b68597a 100644
--- a/Microsoft.Azure.Cosmos/src/ConnectionPolicy.cs
+++ b/Microsoft.Azure.Cosmos/src/ConnectionPolicy.cs
@@ -447,6 +447,14 @@ public TimeSpan? IdleTcpConnectionTimeout
         /// </value>
         /// <remarks>
         /// When the time elapses, the attempt is cancelled and an error is returned. Longer timeouts will delay retries and failures.
+        /// The supplied <see cref="TimeSpan"/> is preserved unchanged on this property. At the transport boundary,
+        /// values in [<see cref="TimeSpan.Zero"/>, 1 second) are treated as 0 (use <see cref="RequestTimeout"/>)
+        /// and values greater than or equal to 1 second are rounded up to the nearest whole second
+        /// (for example, 2.3 seconds becomes 3 seconds).
+        /// Negative values are not recommended and will emit a warning trace. They are preserved
+        /// on this property for backward compatibility; at the transport boundary they are truncated
+        /// to whole seconds and the <c>TransportClient.Options.OpenTimeout</c> getter returns
+        /// <see cref="RequestTimeout"/> for any stored value that is not greater than <see cref="TimeSpan.Zero"/>.
         /// </remarks>
         public TimeSpan? OpenTcpConnectionTimeout
         {
diff --git a/Microsoft.Azure.Cosmos/src/CosmosClientOptions.cs b/Microsoft.Azure.Cosmos/src/CosmosClientOptions.cs
index 38c8997dc0..c25001cc54 100644
--- a/Microsoft.Azure.Cosmos/src/CosmosClientOptions.cs
+++ b/Microsoft.Azure.Cosmos/src/CosmosClientOptions.cs
@@ -12,7 +12,8 @@ namespace Microsoft.Azure.Cosmos
     using System.Net;
     using System.Net.Http;
     using System.Net.Security;
-    using System.Security.Cryptography.X509Certificates;
+    using System.Security.Cryptography.X509Certificates;
+    using Microsoft.Azure.Cosmos.Core.Trace;
     using Microsoft.Azure.Cosmos.FaultInjection;
     using Microsoft.Azure.Cosmos.Fluent;
     using Microsoft.Azure.Documents;
@@ -605,12 +606,40 @@ public TimeSpan? IdleTcpConnectionTimeout
         /// </value>
         /// <remarks>
         /// When the time elapses, the attempt is cancelled and an error is returned. Longer timeouts will delay retries and failures.
+        /// <para>
+        /// The supplied <see cref="TimeSpan"/> is preserved unchanged on this property. At the
+        /// transport boundary the value is converted to whole seconds:
+        /// </para>
+        /// <list type="bullet">
+        /// <item>
+        /// Values in [<see cref="TimeSpan.Zero"/>, 1 second) are treated as 0, causing the
+        /// configured <see cref="RequestTimeout"/> to be used as the open-connection timeout.
+        /// </item>
+        /// <item>
+        /// Values greater than or equal to 1 second are rounded up to the nearest whole second
+        /// (for example, 2.3 seconds becomes 3 seconds).
+        /// </item>
+        /// </list>
+        /// Negative values are not recommended and will emit a warning trace. They are preserved
+        /// on this property for backward compatibility. At the transport boundary they are converted
+        /// to whole seconds via truncation (e.g. −5.7 s → −5) and ultimately reach the
+        /// <c>TransportClient.Options.OpenTimeout</c> getter, which returns
+        /// <see cref="RequestTimeout"/> for any value that is not greater than
+        /// <see cref="TimeSpan.Zero"/>.
         /// </remarks>
         public TimeSpan? OpenTcpConnectionTimeout
         {
             get => this.openTcpConnectionTimeout;
             set
             {
+                if (value.HasValue && value.Value < TimeSpan.Zero)
+                {
+                    DefaultTrace.TraceWarning(
+                        "OpenTcpConnectionTimeout value {0} is negative. Negative values are not recommended; "
+                        + "the TransportClient will fall back to the configured RequestTimeout.",
+                        value.Value);
+                }
+
                 this.openTcpConnectionTimeout = value;
                 this.ValidateDirectTCPSettings();
             }
diff --git a/Microsoft.Azure.Cosmos/src/DocumentClient.cs b/Microsoft.Azure.Cosmos/src/DocumentClient.cs
index c70b0b8668..4f439eec2a 100644
--- a/Microsoft.Azure.Cosmos/src/DocumentClient.cs
+++ b/Microsoft.Azure.Cosmos/src/DocumentClient.cs
@@ -927,7 +927,24 @@ internal virtual void Initialize(Uri serviceEndpoint,
 
                 if (connectionPolicy.OpenTcpConnectionTimeout.HasValue)
                 {
-                    this.openConnectionTimeoutInSeconds = (int)connectionPolicy.OpenTcpConnectionTimeout.Value.TotalSeconds;
+                    // Values in [TimeSpan.Zero, 1 second) become 0 (use RequestTimeout).
+                    // Values >= 1 second round up to the nearest whole second, clamped to int.MaxValue.
+                    // Negative values are truncated via (int)TotalSeconds, preserving pre-PR behavior.
+                    TimeSpan openTcpConnectionTimeout = connectionPolicy.OpenTcpConnectionTimeout.Value;
+
+                    if (openTcpConnectionTimeout < TimeSpan.Zero)
+                    {
+                        this.openConnectionTimeoutInSeconds = (int)openTcpConnectionTimeout.TotalSeconds;
+                    }
+                    else if (openTcpConnectionTimeout < TimeSpan.FromSeconds(1))
+                    {
+                        this.openConnectionTimeoutInSeconds = 0;
+                    }
+                    else
+                    {
+                        double ceilingSeconds = Math.Ceiling(openTcpConnectionTimeout.TotalSeconds);
+                        this.openConnectionTimeoutInSeconds = ceilingSeconds > int.MaxValue ? int.MaxValue : (int)ceilingSeconds;
+                    }
                 }
 
                 if (connectionPolicy.MaxRequestsPerTcpConnection.HasValue)
diff --git a/Microsoft.Azure.Cosmos/src/Fluent/CosmosClientBuilder.cs b/Microsoft.Azure.Cosmos/src/Fluent/CosmosClientBuilder.cs
index d7a9dac0c4..a3a93d5b14 100644
--- a/Microsoft.Azure.Cosmos/src/Fluent/CosmosClientBuilder.cs
+++ b/Microsoft.Azure.Cosmos/src/Fluent/CosmosClientBuilder.cs
@@ -435,6 +435,12 @@ public CosmosClientBuilder WithConnectionModeDirect()
         /// Controls the amount of time allowed for trying to establish a connection.
         /// The default timeout is 5 seconds. Recommended values are greater than or equal to 5 seconds.
         /// When the time elapses, the attempt is cancelled and an error is returned. Longer timeouts will delay retries and failures.
+        /// At the transport boundary, values in [<see cref="TimeSpan.Zero"/>, 1 second) are treated as 0
+        /// (use the configured request timeout). Values greater than or equal to 1 second are rounded up to
+        /// the nearest whole second (for example, 2.3 seconds becomes 3 seconds).
+        /// Negative values are not recommended and will emit a warning trace. They are preserved for
+        /// backward compatibility; at the transport boundary they are truncated to whole seconds and the
+        /// TransportClient returns the configured request timeout for any stored value not greater than zero.
         /// </param>
         /// <param name="maxRequestsPerTcpConnection">
         /// Controls the number of requests allowed simultaneously over a single TCP connection. When more requests are in flight simultaneously, the direct/TCP client will open additional connections.
diff --git a/Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/CosmosClientOptionsUnitTests.cs b/Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/CosmosClientOptionsUnitTests.cs
index 9b3797ba11..27e7989653 100644
--- a/Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/CosmosClientOptionsUnitTests.cs
+++ b/Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/CosmosClientOptionsUnitTests.cs
@@ -921,6 +921,191 @@ public void VerifyGetConnectionPolicyThrowIfDirectTcpSettingAreUsedInGatewayMode
             Assert.ThrowsException<ArgumentException>(() => cosmosClientOptions.MaxTcpConnectionsPerEndpoint = maxTcpConnectionsPerEndpoint);
         }
 
+        [TestMethod]
+        public void OpenTcpConnectionTimeoutNegativeTimeSpanPassesThroughWithWarning()
+        {
+            CosmosClientOptions options = new CosmosClientOptions
+            {
+                ConnectionMode = ConnectionMode.Direct,
+            };
+
+            // Negative values are passed through unchanged (warning is logged but value is preserved).
+            options.OpenTcpConnectionTimeout = TimeSpan.FromMilliseconds(-1);
+            Assert.AreEqual(TimeSpan.FromMilliseconds(-1), options.OpenTcpConnectionTimeout,
+                "Negative value should be preserved unchanged on the property.");
+
+            options.OpenTcpConnectionTimeout = TimeSpan.FromSeconds(-30);
+            Assert.AreEqual(TimeSpan.FromSeconds(-30), options.OpenTcpConnectionTimeout,
+                "Negative value should be preserved unchanged on the property.");
+        }
+
+        [TestMethod]
+        public void OpenTcpConnectionTimeoutZeroIsAllowedAndRoundTripsThroughConnectionPolicy()
+        {
+            CosmosClientOptions options = new CosmosClientOptions
+            {
+                ConnectionMode = ConnectionMode.Direct,
+                OpenTcpConnectionTimeout = TimeSpan.Zero,
+            };
+
+            ConnectionPolicy policy = options.GetConnectionPolicy(clientId: 0);
+            Assert.AreEqual(TimeSpan.Zero, policy.OpenTcpConnectionTimeout);
+        }
+
+        [TestMethod]
+        public void OpenTcpConnectionTimeoutSubSecondNormalizesToZeroInRntbdConfig()
+        {
+            CosmosClientOptions options = new CosmosClientOptions
+            {
+                ConnectionMode = ConnectionMode.Direct,
+                OpenTcpConnectionTimeout = TimeSpan.FromMilliseconds(500),
+            };
+
+            ConnectionPolicy policy = options.GetConnectionPolicy(clientId: 0);
+            CosmosClientBuilder builder = new CosmosClientBuilder(
+                accountEndpoint: AccountEndpoint,
+                authKeyOrResourceToken: MockCosmosUtil.RandomInvalidCorrectlyFormatedAuthKey);
+            CosmosClient cosmosClient = builder.Build(new MockDocumentClient(connectionPolicy: policy));
+
+            Microsoft.Azure.Cosmos.Tracing.TraceData.RntbdConnectionConfig tcpConfig =
+                cosmosClient.ClientConfigurationTraceDatum.RntbdConnectionConfig;
+
+            Assert.AreEqual(
+                0,
+                tcpConfig.ConnectionTimeout,
+                "Sub-second OpenTcpConnectionTimeout must surface as 0 seconds (fall back to request timeout).");
+        }
+
+        [TestMethod]
+        public void OpenTcpConnectionTimeoutExactlyOneSecondPreservedInRntbdConfig()
+        {
+            CosmosClientOptions options = new CosmosClientOptions
+            {
+                ConnectionMode = ConnectionMode.Direct,
+                OpenTcpConnectionTimeout = TimeSpan.FromSeconds(1),
+            };
+
+            ConnectionPolicy policy = options.GetConnectionPolicy(clientId: 0);
+            CosmosClientBuilder builder = new CosmosClientBuilder(
+                accountEndpoint: AccountEndpoint,
+                authKeyOrResourceToken: MockCosmosUtil.RandomInvalidCorrectlyFormatedAuthKey);
+            CosmosClient cosmosClient = builder.Build(new MockDocumentClient(connectionPolicy: policy));
+
+            Microsoft.Azure.Cosmos.Tracing.TraceData.RntbdConnectionConfig tcpConfig =
+                cosmosClient.ClientConfigurationTraceDatum.RntbdConnectionConfig;
+
+            Assert.AreEqual(1, tcpConfig.ConnectionTimeout);
+        }
+
+        [TestMethod]
+        public void OpenTcpConnectionTimeoutWholeSecondsPreservedInRntbdConfig()
+        {
+            CosmosClientOptions options = new CosmosClientOptions
+            {
+                ConnectionMode = ConnectionMode.Direct,
+                OpenTcpConnectionTimeout = TimeSpan.FromSeconds(7),
+            };
+
+            ConnectionPolicy policy = options.GetConnectionPolicy(clientId: 0);
+            CosmosClientBuilder builder = new CosmosClientBuilder(
+                accountEndpoint: AccountEndpoint,
+                authKeyOrResourceToken: MockCosmosUtil.RandomInvalidCorrectlyFormatedAuthKey);
+            CosmosClient cosmosClient = builder.Build(new MockDocumentClient(connectionPolicy: policy));
+
+            Microsoft.Azure.Cosmos.Tracing.TraceData.RntbdConnectionConfig tcpConfig =
+                cosmosClient.ClientConfigurationTraceDatum.RntbdConnectionConfig;
+
+            Assert.AreEqual(7, tcpConfig.ConnectionTimeout);
+        }
+
+        [TestMethod]
+        public void OpenTcpConnectionTimeoutFractionalRoundsUpInRntbdConfig()
+        {
+            CosmosClientOptions options = new CosmosClientOptions
+            {
+                ConnectionMode = ConnectionMode.Direct,
+                OpenTcpConnectionTimeout = TimeSpan.FromSeconds(2.5),
+            };
+
+            ConnectionPolicy policy = options.GetConnectionPolicy(clientId: 0);
+            CosmosClientBuilder builder = new CosmosClientBuilder(
+                accountEndpoint: AccountEndpoint,
+                authKeyOrResourceToken: MockCosmosUtil.RandomInvalidCorrectlyFormatedAuthKey);
+            CosmosClient cosmosClient = builder.Build(new MockDocumentClient(connectionPolicy: policy));
+
+            Microsoft.Azure.Cosmos.Tracing.TraceData.RntbdConnectionConfig tcpConfig =
+                cosmosClient.ClientConfigurationTraceDatum.RntbdConnectionConfig;
+
+            Assert.AreEqual(
+                3,
+                tcpConfig.ConnectionTimeout,
+                "Fractional OpenTcpConnectionTimeout (>= 1s) rounds up to the nearest whole second at the transport boundary.");
+        }
+
+        [TestMethod]
+        public void OpenTcpConnectionTimeoutJustOverOneSecondRoundsUpInRntbdConfig()
+        {
+            CosmosClientOptions options = new CosmosClientOptions
+            {
+                ConnectionMode = ConnectionMode.Direct,
+                OpenTcpConnectionTimeout = TimeSpan.FromMilliseconds(1001),
+            };
+
+            ConnectionPolicy policy = options.GetConnectionPolicy(clientId: 0);
+            CosmosClientBuilder builder = new CosmosClientBuilder(
+                accountEndpoint: AccountEndpoint,
+                authKeyOrResourceToken: MockCosmosUtil.RandomInvalidCorrectlyFormatedAuthKey);
+            CosmosClient cosmosClient = builder.Build(new MockDocumentClient(connectionPolicy: policy));
+
+            Microsoft.Azure.Cosmos.Tracing.TraceData.RntbdConnectionConfig tcpConfig =
+                cosmosClient.ClientConfigurationTraceDatum.RntbdConnectionConfig;
+
+            Assert.AreEqual(
+                2,
+                tcpConfig.ConnectionTimeout,
+                "1.001s rounds up to 2s at the transport boundary.");
+        }
+
+        [TestMethod]
+        public void OpenTcpConnectionTimeoutFractionalPreservedOnConnectionPolicyTimeSpan()
+        {
+            TimeSpan customerSupplied = TimeSpan.FromSeconds(2.5);
+            CosmosClientOptions options = new CosmosClientOptions
+            {
+                ConnectionMode = ConnectionMode.Direct,
+                OpenTcpConnectionTimeout = customerSupplied,
+            };
+
+            Assert.AreEqual(
+                customerSupplied,
+                options.OpenTcpConnectionTimeout,
+                "CosmosClientOptions preserves the supplied TimeSpan unchanged.");
+
+            ConnectionPolicy policy = options.GetConnectionPolicy(clientId: 0);
+            Assert.AreEqual(
+                customerSupplied,
+                policy.OpenTcpConnectionTimeout,
+                "ConnectionPolicy preserves the supplied TimeSpan unchanged.");
+        }
+
+        [TestMethod]
+        public void WithConnectionModeDirectNegativeOpenTcpTimeoutPassesThroughUnchanged()
+        {
+            CosmosClientOptions options = new CosmosClientOptions
+            {
+                ConnectionMode = ConnectionMode.Direct,
+            };
+
+            // Negative values are preserved on the property and round-trip through ConnectionPolicy.
+            options.OpenTcpConnectionTimeout = TimeSpan.FromSeconds(-1);
+            Assert.AreEqual(TimeSpan.FromSeconds(-1), options.OpenTcpConnectionTimeout,
+                "Negative openTcpConnectionTimeout should be preserved unchanged.");
+
+            ConnectionPolicy policy = options.GetConnectionPolicy(clientId: 0);
+            Assert.AreEqual(TimeSpan.FromSeconds(-1), policy.OpenTcpConnectionTimeout,
+                "Negative value should round-trip through ConnectionPolicy unchanged.");
+        }
+
         [TestMethod]
         public void VerifyHttpClientFactoryBlockedWithConnectionLimit()
         {
diff --git a/changelog.md b/changelog.md
index db34364e9c..d75ee5165a 100644
--- a/changelog.md
+++ b/changelog.md
@@ -44,6 +44,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 #### Bugs Fixed
 
+- [5873](https://github.com/Azure/azure-cosmos-dotnet-v3/pull/5873) Direct: Fixes silent truncation of `OpenTcpConnectionTimeout`. Sub-second values in [0, 1s) are now explicitly normalized to 0 and fractional values ≥ 1 second are rounded up to the nearest whole second (for example, 2.3s becomes 3s). Negative values emit a warning trace but are left unchanged for backward compatibility.
 - [5783](https://github.com/Azure/azure-cosmos-dotnet-v3/pull/5783) Container: Fixes SemanticRerankAsync TypeLoadException in derived classes
 
 ### <a name="3.60.0"/> [3.60.0](https://www.nuget.org/packages/Microsoft.Azure.Cosmos/3.60.0) - 2026-5-18