diff --git a/src/OpenTelemetry.OpAmp.Client/.publicApi/PublicAPI.Unshipped.txt b/src/OpenTelemetry.OpAmp.Client/.publicApi/PublicAPI.Unshipped.txt
index fe03eb54b3..4517bb6327 100644
--- a/src/OpenTelemetry.OpAmp.Client/.publicApi/PublicAPI.Unshipped.txt
+++ b/src/OpenTelemetry.OpAmp.Client/.publicApi/PublicAPI.Unshipped.txt
@@ -23,6 +23,8 @@ OpenTelemetry.OpAmp.Client.Messages.RemoteConfigMessage.ConfigHash.get -> System
 OpenTelemetry.OpAmp.Client.OpAmpClient
 OpenTelemetry.OpAmp.Client.OpAmpClient.Dispose() -> void
 OpenTelemetry.OpAmp.Client.OpAmpClient.OpAmpClient(System.Action<OpenTelemetry.OpAmp.Client.Settings.OpAmpClientSettings!>? configure = null) -> void
+OpenTelemetry.OpAmp.Client.OpAmpClient.SendCustomCapabilitiesAsync(System.Collections.Generic.IEnumerable<string!>! capabilities, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> System.Threading.Tasks.Task!
+OpenTelemetry.OpAmp.Client.OpAmpClient.SendCustomMessageAsync(string! capability, string! type, System.ReadOnlyMemory<byte> data, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> System.Threading.Tasks.Task!
 OpenTelemetry.OpAmp.Client.OpAmpClient.SendEffectiveConfigAsync(System.Collections.Generic.IEnumerable<OpenTelemetry.OpAmp.Client.Messages.EffectiveConfigFile!>! files, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> System.Threading.Tasks.Task!
 OpenTelemetry.OpAmp.Client.OpAmpClient.StartAsync(System.Threading.CancellationToken token = default(System.Threading.CancellationToken)) -> System.Threading.Tasks.Task!
 OpenTelemetry.OpAmp.Client.OpAmpClient.StopAsync(System.Threading.CancellationToken token = default(System.Threading.CancellationToken)) -> System.Threading.Tasks.Task!
diff --git a/src/OpenTelemetry.OpAmp.Client/CHANGELOG.md b/src/OpenTelemetry.OpAmp.Client/CHANGELOG.md
index 1549d69648..5ef588bb35 100644
--- a/src/OpenTelemetry.OpAmp.Client/CHANGELOG.md
+++ b/src/OpenTelemetry.OpAmp.Client/CHANGELOG.md
@@ -5,6 +5,9 @@
 * Add agent effective config reporting.
   ([#3716](https://github.com/open-telemetry/opentelemetry-dotnet-contrib/pull/3716))
 
+* Add ability to send custom messages.
+  ([#3809](https://github.com/open-telemetry/opentelemetry-dotnet-contrib/pull/3809))
+
 ## 0.1.0-alpha.4
 
 Released 2026-Jan-14
diff --git a/src/OpenTelemetry.OpAmp.Client/Internal/FrameBuilder.cs b/src/OpenTelemetry.OpAmp.Client/Internal/FrameBuilder.cs
index 5231b97c7e..002794f94e 100644
--- a/src/OpenTelemetry.OpAmp.Client/Internal/FrameBuilder.cs
+++ b/src/OpenTelemetry.OpAmp.Client/Internal/FrameBuilder.cs
@@ -156,6 +156,30 @@ IFrameBuilder IFrameBuilder.AddCapabilities()
         return this;
     }
 
+    IFrameBuilder IFrameBuilder.AddCustomCapabilities(IEnumerable<string> capabilities)
+    {
+        this.EnsureInitialized();
+
+        this.currentMessage.CustomCapabilities = new CustomCapabilities();
+        this.currentMessage.CustomCapabilities.Capabilities.Add(capabilities);
+
+        return this;
+    }
+
+    IFrameBuilder IFrameBuilder.AddCustomMessage(string capability, string type, ReadOnlyMemory<byte> data)
+    {
+        this.EnsureInitialized();
+
+        this.currentMessage.CustomMessage = new CustomMessage
+        {
+            Capability = capability,
+            Type = type,
+            Data = ByteString.CopyFrom(data.Span),
+        };
+
+        return this;
+    }
+
     IFrameBuilder IFrameBuilder.AddEffectiveConfig(IEnumerable<EffectiveConfigFile> files)
     {
         this.EnsureInitialized();
diff --git a/src/OpenTelemetry.OpAmp.Client/Internal/FrameDispatcher.cs b/src/OpenTelemetry.OpAmp.Client/Internal/FrameDispatcher.cs
index e08785bfdc..80ff4eab94 100644
--- a/src/OpenTelemetry.OpAmp.Client/Internal/FrameDispatcher.cs
+++ b/src/OpenTelemetry.OpAmp.Client/Internal/FrameDispatcher.cs
@@ -86,6 +86,34 @@ AgentToServer BuildEffectiveConfigMessage(FrameBuilder fb)
         }
     }
 
+    public async Task DispatchCustomCapabilitiesAsync(IEnumerable<string> capabilities, CancellationToken token)
+    {
+        await this.DispatchFrameAsync(
+            BuildCustomCapabilitiesMessage,
+            OpAmpClientEventSource.Log.SendingCustomCapabilitiesMessage,
+            OpAmpClientEventSource.Log.SendCustomCapabilitiesMessageException,
+            token).ConfigureAwait(false);
+
+        AgentToServer BuildCustomCapabilitiesMessage(FrameBuilder fb)
+        {
+            return fb.StartBaseMessage().AddCustomCapabilities(capabilities).Build();
+        }
+    }
+
+    public async Task DispatchCustomMessageAsync(string capability, string type, ReadOnlyMemory<byte> data, CancellationToken token)
+    {
+        await this.DispatchFrameAsync(
+            BuildCustomMessageMessage,
+            OpAmpClientEventSource.Log.SendingCustomMessageMessage,
+            OpAmpClientEventSource.Log.SendCustomMessageMessageException,
+            token).ConfigureAwait(false);
+
+        AgentToServer BuildCustomMessageMessage(FrameBuilder fb)
+        {
+            return fb.StartBaseMessage().AddCustomMessage(capability, type, data).Build();
+        }
+    }
+
     public void Dispose()
     {
         this.syncRoot.Dispose();
diff --git a/src/OpenTelemetry.OpAmp.Client/Internal/IFrameBuilder.cs b/src/OpenTelemetry.OpAmp.Client/Internal/IFrameBuilder.cs
index 57e99e62e4..2451ec022f 100644
--- a/src/OpenTelemetry.OpAmp.Client/Internal/IFrameBuilder.cs
+++ b/src/OpenTelemetry.OpAmp.Client/Internal/IFrameBuilder.cs
@@ -17,7 +17,11 @@ internal interface IFrameBuilder
 
     IFrameBuilder AddCapabilities();
 
+    IFrameBuilder AddCustomCapabilities(IEnumerable<string> capabilities);
+
     IFrameBuilder AddEffectiveConfig(IEnumerable<EffectiveConfigFile> files);
 
+    IFrameBuilder AddCustomMessage(string capability, string type, ReadOnlyMemory<byte> data);
+
     AgentToServer Build();
 }
diff --git a/src/OpenTelemetry.OpAmp.Client/Internal/OpAmpClientEventSource.cs b/src/OpenTelemetry.OpAmp.Client/Internal/OpAmpClientEventSource.cs
index eaa16cdeae..c2698301f7 100644
--- a/src/OpenTelemetry.OpAmp.Client/Internal/OpAmpClientEventSource.cs
+++ b/src/OpenTelemetry.OpAmp.Client/Internal/OpAmpClientEventSource.cs
@@ -26,12 +26,16 @@ internal class OpAmpClientEventSource : EventSource
     private const int EventIdSendingHeartbeatMessage = 1_001;
     private const int EventIdSendingAgentDisconnectMessage = 1_002;
     private const int EventIdSendingEffectiveConfigMessage = 1_003;
+    private const int EventIdSendingCustomCapabilitiesMessage = 1_004;
+    private const int EventIdSendingCustomMessageMessage = 1_005;
 
     // FrameDispatcher error messages 1100-1199
     private const int EventIdFailedToSendIdentificationMessage = 1_100;
     private const int EventIdFailedToSendHeartbeatMessage = 1_101;
     private const int EventIdFailedToSendAgentDisconnectMessage = 1_102;
     private const int EventIdFailedToSendEffectiveConfigMessage = 1_103;
+    private const int EventIdFailedToSendCustomCapabilitiesMessage = 1_104;
+    private const int EventIdFailedToSendCustomMessageMessage = 1_105;
 
     [Event(EventIdInvalidWsFrame, Message = "Received invalid WebSocket frame header: {0}. Dropping the frame.", Level = EventLevel.Warning)]
     public void InvalidWsFrame(string errorMessage)
@@ -113,6 +117,18 @@ public void SendingEffectiveConfigMessage()
         this.WriteEvent(EventIdSendingEffectiveConfigMessage);
     }
 
+    [Event(EventIdSendingCustomCapabilitiesMessage, Message = "Sending custom capabilities message.", Level = EventLevel.Informational)]
+    public void SendingCustomCapabilitiesMessage()
+    {
+        this.WriteEvent(EventIdSendingCustomCapabilitiesMessage);
+    }
+
+    [Event(EventIdSendingCustomMessageMessage, Message = "Sending custom message.", Level = EventLevel.Informational)]
+    public void SendingCustomMessageMessage()
+    {
+        this.WriteEvent(EventIdSendingCustomMessageMessage);
+    }
+
     [NonEvent]
     public void SendIdentificationMessageException(Exception ex)
     {
@@ -172,4 +188,34 @@ public void FailedToSendEffectiveConfigMessage(string exception)
     {
         this.WriteEvent(EventIdFailedToSendEffectiveConfigMessage, exception);
     }
+
+    [NonEvent]
+    public void SendCustomCapabilitiesMessageException(Exception ex)
+    {
+        if (!this.IsEnabled(EventLevel.Error, EventKeywords.All))
+        {
+            this.FailedToSendCustomCapabilitiesMessage(ex.ToInvariantString());
+        }
+    }
+
+    [Event(EventIdFailedToSendCustomCapabilitiesMessage, Message = "Failed to send custom capabilities message: {0}", Level = EventLevel.Error)]
+    public void FailedToSendCustomCapabilitiesMessage(string exception)
+    {
+        this.WriteEvent(EventIdFailedToSendCustomCapabilitiesMessage, exception);
+    }
+
+    [NonEvent]
+    public void SendCustomMessageMessageException(Exception ex)
+    {
+        if (!this.IsEnabled(EventLevel.Error, EventKeywords.All))
+        {
+            this.FailedToSendCustomMessageMessage(ex.ToInvariantString());
+        }
+    }
+
+    [Event(EventIdFailedToSendCustomMessageMessage, Message = "Failed to send a custom message: {0}", Level = EventLevel.Error)]
+    public void FailedToSendCustomMessageMessage(string exception)
+    {
+        this.WriteEvent(EventIdFailedToSendCustomMessageMessage, exception);
+    }
 }
diff --git a/src/OpenTelemetry.OpAmp.Client/OpAmpClient.cs b/src/OpenTelemetry.OpAmp.Client/OpAmpClient.cs
index 722fc58d77..0b1a3233ce 100644
--- a/src/OpenTelemetry.OpAmp.Client/OpAmpClient.cs
+++ b/src/OpenTelemetry.OpAmp.Client/OpAmpClient.cs
@@ -123,6 +123,30 @@ public Task SendEffectiveConfigAsync(IEnumerable<EffectiveConfigFile> files, Can
         return this.dispatcher.DispatchEffectiveConfigAsync(files, cancellationToken);
     }
 
+    /// <summary>
+    /// Reports custom capabilities supported by the agent.
+    /// </summary>
+    /// <param name="capabilities">Capabilities list.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>A task that represents the asynchronous send operation.</returns>
+    public Task SendCustomCapabilitiesAsync(IEnumerable<string> capabilities, CancellationToken cancellationToken = default)
+    {
+        return this.dispatcher.DispatchCustomCapabilitiesAsync(capabilities, cancellationToken);
+    }
+
+    /// <summary>
+    /// Sends a custom message related to a supported custom capability.
+    /// </summary>
+    /// <param name="capability">Capability that matches a reported custom capability.</param>
+    /// <param name="type">Type of message within the capability.</param>
+    /// <param name="data">Contents of the message.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>A task that represents the asynchronous send operation.</returns>
+    public Task SendCustomMessageAsync(string capability, string type, ReadOnlyMemory<byte> data, CancellationToken cancellationToken = default)
+    {
+        return this.dispatcher.DispatchCustomMessageAsync(capability, type, data, cancellationToken);
+    }
+
     /// <summary>
     /// Disposes the OpAmpClient instance and releases all associated resources.
     /// </summary>
diff --git a/test/OpenTelemetry.OpAmp.Client.Tests/OpAmpClientTests.cs b/test/OpenTelemetry.OpAmp.Client.Tests/OpAmpClientTests.cs
index ab4fe4f15b..1f21ffd548 100644
--- a/test/OpenTelemetry.OpAmp.Client.Tests/OpAmpClientTests.cs
+++ b/test/OpenTelemetry.OpAmp.Client.Tests/OpAmpClientTests.cs
@@ -249,6 +249,69 @@ internal async Task SendsEffectiveConfigFile_FromFile()
         Assert.Equal(fileContentType, actualConfig.ContentType);
     }
 
+    [Fact]
+    internal async Task SendsCustomCapabilities()
+    {
+        // Setup OpAMP server
+        using var opAmpServer = new OpAmpFakeHttpServer(false);
+        var opAmpEndpoint = opAmpServer.Endpoint;
+
+        using var client = new OpAmpClient(o =>
+        {
+            o.ServerUrl = opAmpEndpoint;
+        });
+
+        // Setup content
+        string[] capabilities = ["custom-c1", "custom-c2", "custom-c3"];
+
+        // Act
+        await client.StartAsync();
+        await client.SendCustomCapabilitiesAsync(capabilities);
+        await client.StopAsync();
+
+        // Assert received frames
+        var frames = opAmpServer.GetFrames();
+        var actualCapabilities = frames[1].CustomCapabilities.Capabilities;
+
+        Assert.Equal(3, frames.Count); // 3 frames: 1 identification, 2 custom capabilities, 3 disconnect
+        Assert.Equal(capabilities, actualCapabilities);
+    }
+
+    [Fact]
+    internal async Task SendsCustomMessage()
+    {
+        // Setup OpAMP server
+        using var opAmpServer = new OpAmpFakeHttpServer(false);
+        var opAmpEndpoint = opAmpServer.Endpoint;
+
+        using var client = new OpAmpClient(o =>
+        {
+            o.ServerUrl = opAmpEndpoint;
+        });
+
+        // Setup content
+        const string capability = "custom-c1";
+        const string type = "custom-type-1";
+        const string messageContent = "a custom message contents";
+        var message = Encoding.UTF8.GetBytes(messageContent);
+
+        // Act
+        await client.StartAsync();
+        await client.SendCustomMessageAsync(capability, type, message);
+        await client.StopAsync();
+
+        // Assert received frames
+        var frames = opAmpServer.GetFrames();
+        var actualCapability = frames[1].CustomMessage.Capability;
+        var actualType = frames[1].CustomMessage.Type;
+        var actualMessageContent = frames[1].CustomMessage.Data.ToStringUtf8();
+
+        Assert.Equal(3, frames.Count); // 3 frames: 1 identification, 2 custom message, 3 disconnect
+        Assert.Equal(capability, actualCapability);
+        Assert.Equal(type, actualType);
+        Assert.Equal(messageContent, actualMessageContent);
+    }
+
     internal class CapabilityTestData
         : TheoryData<Action<OpAmpClientSettings>, IEnumerable<AgentCapabilities>, IEnumerable<AgentCapabilities>>
     {