diff --git a/src/DocumentDbTests/Bugs/Bug_4223_update_partitioned_composite_pk.cs b/src/DocumentDbTests/Bugs/Bug_4223_update_partitioned_composite_pk.cs
new file mode 100644
index 0000000000..333fb23750
--- /dev/null
+++ b/src/DocumentDbTests/Bugs/Bug_4223_update_partitioned_composite_pk.cs
@@ -0,0 +1,121 @@
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Marten;
+using Marten.Testing.Harness;
+using Shouldly;
+using Xunit;
+
+namespace DocumentDbTests.Bugs;
+
+public class Bug4223Snapshot
+{
+    public string Id { get; set; } = "";
+    public DateOnly Date { get; set; }
+    public decimal Quantity { get; set; }
+}
+
+public class Bug_4223_update_partitioned_composite_pk : OneOffConfigurationsContext
+{
+    public Bug_4223_update_partitioned_composite_pk()
+    {
+        _schemaName = "bug4223";
+    }
+
+    [Fact]
+    public async Task update_with_same_id_different_partition_key()
+    {
+        StoreOptions(opts =>
+        {
+            opts.Schema.For<Bug4223Snapshot>()
+                .Identity(x => x.Id)
+                .Duplicate(x => x.Date)
+                .PartitionOn(x => x.Date, x =>
+                {
+                    x.ByRange()
+                        .AddRange("q1_2026", new DateOnly(2026, 1, 1), new DateOnly(2026, 4, 1))
+                        .AddRange("q2_2026", new DateOnly(2026, 4, 1), new DateOnly(2026, 7, 1));
+                });
+        });
+
+        await theStore.Storage.ApplyAllConfiguredChangesToDatabaseAsync();
+
+        // Insert two documents with same id, different date (valid per composite PK)
+        var d1 = new Bug4223Snapshot { Id = "sensor-1", Date = new DateOnly(2026, 1, 15), Quantity = 100 };
+        var d2 = new Bug4223Snapshot { Id = "sensor-1", Date = new DateOnly(2026, 3, 15), Quantity = 200 };
+
+        await using (var session = theStore.LightweightSession())
+        {
+            session.Store(d1);
+            session.Store(d2);
+            await session.SaveChangesAsync();
+        }
+
+        // Update only the first one — should NOT affect the second
+        await using (var session = theStore.LightweightSession())
+        {
+            d1.Quantity = 999;
+            session.Update(d1);
+            await session.SaveChangesAsync();
+        }
+
+        // Verify: d1 updated, d2 unchanged
+        await using (var query = theStore.QuerySession())
+        {
+            var results = await query.Query<Bug4223Snapshot>()
+                .Where(x => x.Id == "sensor-1")
+                .OrderBy(x => x.Date)
+                .ToListAsync();
+
+            results.Count.ShouldBe(2);
+            results[0].Quantity.ShouldBe(999m);  // updated
+            results[1].Quantity.ShouldBe(200m);  // unchanged
+        }
+    }
+
+    [Fact]
+    public async Task upsert_with_same_id_different_partition_key()
+    {
+        StoreOptions(opts =>
+        {
+            opts.Schema.For<Bug4223Snapshot>()
+                .Identity(x => x.Id)
+                .Duplicate(x => x.Date)
+                .PartitionOn(x => x.Date, x =>
+                {
+                    x.ByRange()
+                        .AddRange("q1_2026", new DateOnly(2026, 1, 1), new DateOnly(2026, 4, 1))
+                        .AddRange("q2_2026", new DateOnly(2026, 4, 1), new DateOnly(2026, 7, 1));
+                });
+        });
+
+        await theStore.Storage.ApplyAllConfiguredChangesToDatabaseAsync();
+
+        // Store two documents with same id, different dates
+        await using (var session = theStore.LightweightSession())
+        {
+            session.Store(new Bug4223Snapshot { Id = "sensor-1", Date = new DateOnly(2026, 1, 15), Quantity = 100 });
+            session.Store(new Bug4223Snapshot { Id = "sensor-1", Date = new DateOnly(2026, 3, 15), Quantity = 200 });
+            await session.SaveChangesAsync();
+        }
+
+        // Upsert the first one with new quantity
+        await using (var session = theStore.LightweightSession())
+        {
+            session.Store(new Bug4223Snapshot { Id = "sensor-1", Date = new DateOnly(2026, 1, 15), Quantity = 777 });
+            await session.SaveChangesAsync();
+        }
+
+        await using (var query = theStore.QuerySession())
+        {
+            var results = await query.Query<Bug4223Snapshot>()
+                .Where(x => x.Id == "sensor-1")
+                .OrderBy(x => x.Date)
+                .ToListAsync();
+
+            results.Count.ShouldBe(2);
+            results[0].Quantity.ShouldBe(777m);  // upserted
+            results[1].Quantity.ShouldBe(200m);  // unchanged
+        }
+    }
+}
diff --git a/src/Marten/Storage/UpdateFunction.cs b/src/Marten/Storage/UpdateFunction.cs
index 1c821464b4..f23f019d17 100644
--- a/src/Marten/Storage/UpdateFunction.cs
+++ b/src/Marten/Storage/UpdateFunction.cs
@@ -15,8 +15,8 @@ protected override void writeFunction(TextWriter writer, string argList, string
         string updates)
     {
         var statement = updates.Contains("where")
-            ? $"UPDATE {_tableName} SET {updates} and id = docId;"
-            : $"UPDATE {_tableName} SET {updates} where id = docId;";
+            ? $"UPDATE {_tableName} SET {updates} and id = docId{_andPartitionWhereClause};"
+            : $"UPDATE {_tableName} SET {updates} where id = docId{_andPartitionWhereClause};";
 
         if (_mapping.Metadata.Revision.Enabled)
         {
@@ -29,7 +29,7 @@ protected override void writeFunction(TextWriter writer, string argList, string
   current_version INTEGER;
 BEGIN
   if revision <= 1 then
-    SELECT mt_version FROM {_tableName.QualifiedName} into current_version WHERE id = docId {_andTenantWhereClause};
+    SELECT mt_version FROM {_tableName.QualifiedName} into current_version WHERE id = docId {_andTenantWhereClause}{_andPartitionWhereClause};
     if current_version is not null then
       revision = current_version + 1;
     end if;
@@ -37,7 +37,7 @@ protected override void writeFunction(TextWriter writer, string argList, string
 
   {statement}
 
-  SELECT mt_version FROM {_tableName} into final_version WHERE id = docId {_andTenantWhereClause};
+  SELECT mt_version FROM {_tableName} into final_version WHERE id = docId {_andTenantWhereClause}{_andPartitionWhereClause};
   RETURN final_version;
 END;
 $function$;
@@ -54,7 +54,7 @@ protected override void writeFunction(TextWriter writer, string argList, string
 BEGIN
   {statement}
 
-  SELECT mt_version FROM {_tableName} into final_version WHERE id = docId {_andTenantWhereClause};
+  SELECT mt_version FROM {_tableName} into final_version WHERE id = docId {_andTenantWhereClause}{_andPartitionWhereClause};
   RETURN final_version;
 END;
 $function$;
diff --git a/src/Marten/Storage/UpsertFunction.cs b/src/Marten/Storage/UpsertFunction.cs
index 7eb73e640f..b0f89035f3 100644
--- a/src/Marten/Storage/UpsertFunction.cs
+++ b/src/Marten/Storage/UpsertFunction.cs
@@ -24,6 +24,7 @@ internal class UpsertFunction: Function
 
     public readonly IList<UpsertArgument> Arguments = new List<UpsertArgument>();
     protected readonly string _primaryKeyFields;
+    protected string _andPartitionWhereClause = "";
     protected readonly string _versionSourceTable;
     protected readonly string _versionColumnName;
     protected readonly string _tenantVersionWhereClause;
@@ -112,6 +113,29 @@ public UpsertFunction(DocumentMapping mapping, DbObjectName? identifier = null,
         }
 
         _primaryKeyFields = table.Columns.Where(x => x.IsPrimaryKey).Select(x => x.Name).Join(", ");
+
+        // Build WHERE clause fragment for partition columns (columns that are part of the PK
+        // but are not 'id' or 'tenant_id'). These must be included in UPDATE WHERE and
+        // SELECT WHERE clauses for partitioned tables to correctly target a single row.
+        if (table.Partitioning != null)
+        {
+            var partitionClauses = new List<string>();
+            foreach (var colName in table.Partitioning.Columns)
+            {
+                // Find the matching argument for this partition column
+                var arg = Arguments.FirstOrDefault(a =>
+                    a.Column.Equals(colName, StringComparison.OrdinalIgnoreCase));
+                if (arg != null)
+                {
+                    partitionClauses.Add($"{colName} = {arg.Arg}");
+                }
+            }
+
+            if (partitionClauses.Any())
+            {
+                _andPartitionWhereClause = " and " + partitionClauses.Join(" and ");
+            }
+        }
     }
 
     public void AddIfActive(MetadataColumn column)
@@ -200,7 +224,7 @@ protected virtual void writeFunction(TextWriter writer, string argList, string s
   current_version INTEGER;
 BEGIN
 
-SELECT {_versionColumnName} into current_version FROM {_versionSourceTable} WHERE id = docId {_andTenantVersionWhereClause};
+SELECT {_versionColumnName} into current_version FROM {_versionSourceTable} WHERE id = docId {_andTenantVersionWhereClause}{_andPartitionWhereClause};
 if revision = 0 then
   if current_version is not null then
     {revisionModification}
@@ -219,7 +243,7 @@ protected virtual void writeFunction(TextWriter writer, string argList, string s
   ON CONFLICT ({_primaryKeyFields})
   DO UPDATE SET {updates};
 
-  SELECT mt_version into final_version FROM {_tableName.QualifiedName} WHERE id = docId {_andTenantWhereClause};
+  SELECT mt_version into final_version FROM {_tableName.QualifiedName} WHERE id = docId {_andTenantWhereClause}{_andPartitionWhereClause};
   RETURN final_version;
 END;
 $function$;
@@ -239,7 +263,7 @@ ON CONFLICT ({_primaryKeyFields})
   ON CONFLICT ({_primaryKeyFields})
   DO UPDATE SET {updates};
 
-  SELECT mt_version FROM {_tableName.QualifiedName} into final_version WHERE id = docId {_andTenantWhereClause};
+  SELECT mt_version FROM {_tableName.QualifiedName} into final_version WHERE id = docId {_andTenantWhereClause}{_andPartitionWhereClause};
   RETURN final_version;
 END;
 $function$;