diff --git a/src/EventSourcingTests/Bugs/Bug_4270_global_projection_preserves_event_tenant.cs b/src/EventSourcingTests/Bugs/Bug_4270_global_projection_preserves_event_tenant.cs
new file mode 100644
index 0000000000..112c4b8d0e
--- /dev/null
+++ b/src/EventSourcingTests/Bugs/Bug_4270_global_projection_preserves_event_tenant.cs
@@ -0,0 +1,154 @@
+using System;
+using System.Collections.Concurrent;
+using System.Linq;
+using System.Threading.Tasks;
+using JasperFx;
+using JasperFx.Events;
+using JasperFx.Events.Projections;
+using Marten;
+using Marten.Events;
+using Marten.Events.Aggregation;
+using Marten.Events.Projections;
+using Marten.Storage;
+using Marten.Testing.Harness;
+using Shouldly;
+using Xunit;
+
+namespace EventSourcingTests.Bugs;
+
+/// <summary>
+/// Regression coverage for https://github.com/JasperFx/marten/issues/4270.
+///
+/// When a SingleStreamProjection is registered with AddGlobalProjection on a
+/// store with TenancyStyle.Conjoined, the events the projection consumes should
+/// preserve the tenant id under which they were appended. Prior to the fix,
+/// GlobalEventAppenderDecorator overwrote every IEvent.TenantId on the in-flight
+/// stream with the default tenant id before projections ran, so
+/// Create(IEvent&lt;T&gt;) / Apply(IEvent&lt;T&gt;, TDoc) convention methods that read
+/// @event.TenantId saw "*DEFAULT*" instead of the real tenant under which the
+/// events had been appended. The projection document still stores single-tenanted
+/// (that is the documented purpose of AddGlobalProjection), but the in-flight
+/// IEvent seen by convention methods should reflect the original session tenant.
+/// </summary>
+public class Bug_4270_global_projection_preserves_event_tenant : OneOffConfigurationsContext
+{
+    public Bug_4270_global_projection_preserves_event_tenant()
+    {
+        Bug4270TenantCapturer.Reset();
+    }
+
+    [Fact]
+    public async Task event_tenant_id_is_preserved_in_create_apply_convention_methods()
+    {
+        StoreOptions(opts =>
+        {
+            opts.Events.TenancyStyle = TenancyStyle.Conjoined;
+            opts.Events.StreamIdentity = StreamIdentity.AsString;
+            opts.Policies.AllDocumentsAreMultiTenanted();
+            opts.Projections.AddGlobalProjection(
+                new Bug4270OrderSummaryProjection(),
+                ProjectionLifecycle.Inline);
+        });
+
+        var streamKey = $"order-{Guid.NewGuid():N}";
+        const string tenant = "acme";
+
+        await using var session = theStore.LightweightSession(tenant);
+        session.Events.StartStream(streamKey,
+            new Bug4270OrderCreated("cust-1", 99.50m),
+            new Bug4270OrderShipped());
+        await session.SaveChangesAsync();
+
+        // Inside Create / Apply, @event.TenantId should be the original tenant
+        // under which the events were appended, NOT the default tenant.
+        Bug4270TenantCapturer.TenantIdsSeen.ShouldContain(tenant);
+        Bug4270TenantCapturer.TenantIdsSeen.ShouldNotContain(StorageConstants.DefaultTenantId);
+
+        // The projected document should be stored single-tenanted (i.e., in the default
+        // tenant), but should have captured the original tenant on the document itself.
+        await using var query = theStore.QuerySession();
+        var doc = await query.LoadAsync<Bug4270OrderSummary>(streamKey);
+        doc.ShouldNotBeNull();
+        doc.TenantId.ShouldBe(tenant);
+        doc.Status.ShouldBe("Shipped");
+    }
+
+    [Fact]
+    public async Task projection_storage_is_still_single_tenanted_after_fix()
+    {
+        // AddGlobalProjection is documented to store the aggregate (and its stream)
+        // under the default tenant — that is the whole point of "global within
+        // conjoined tenancy". Assert that preserving the original tenant on
+        // in-flight IEvent objects doesn't accidentally break that guarantee.
+        StoreOptions(opts =>
+        {
+            opts.Events.TenancyStyle = TenancyStyle.Conjoined;
+            opts.Events.StreamIdentity = StreamIdentity.AsString;
+            opts.Policies.AllDocumentsAreMultiTenanted();
+            opts.Projections.AddGlobalProjection(
+                new Bug4270OrderSummaryProjection(),
+                ProjectionLifecycle.Inline);
+        });
+
+        var streamKey = $"order-{Guid.NewGuid():N}";
+        const string tenant = "acme";
+
+        await using var session = theStore.LightweightSession(tenant);
+        session.Events.StartStream(streamKey, new Bug4270OrderCreated("cust-2", 10m));
+        await session.SaveChangesAsync();
+
+        // Projection document should be reachable from the default tenant query
+        // session, even though we appended under "acme".
+        await using var defaultQuery = theStore.QuerySession();
+        var doc = await defaultQuery.LoadAsync<Bug4270OrderSummary>(streamKey);
+        doc.ShouldNotBeNull();
+
+        // And Create() should have seen the ORIGINAL tenant "acme" on the event,
+        // not "*DEFAULT*".
+        doc.TenantId.ShouldBe(tenant);
+    }
+}
+
+// ─────────────────────────── fixtures ───────────────────────────
+
+public record Bug4270OrderCreated(string CustomerId, decimal TotalAmount);
+public record Bug4270OrderShipped;
+
+public class Bug4270OrderSummary
+{
+    public string Id { get; set; } = default!;
+    public string TenantId { get; set; } = default!;
+    public string CustomerId { get; set; } = default!;
+    public decimal TotalAmount { get; set; }
+    public string Status { get; set; } = default!;
+}
+
+public class Bug4270OrderSummaryProjection : SingleStreamProjection<Bug4270OrderSummary, string>
+{
+    public static Bug4270OrderSummary Create(IEvent<Bug4270OrderCreated> @event)
+    {
+        Bug4270TenantCapturer.TenantIdsSeen.Add(@event.TenantId!);
+        return new Bug4270OrderSummary
+        {
+            Id = @event.StreamKey!,
+            TenantId = @event.TenantId!,
+            CustomerId = @event.Data.CustomerId,
+            TotalAmount = @event.Data.TotalAmount,
+            Status = "Created"
+        };
+    }
+
+    public static Bug4270OrderSummary Apply(IEvent<Bug4270OrderShipped> @event, Bug4270OrderSummary item)
+    {
+        Bug4270TenantCapturer.TenantIdsSeen.Add(@event.TenantId!);
+        item.Status = "Shipped";
+        return item;
+    }
+}
+
+internal static class Bug4270TenantCapturer
+{
+    public static ConcurrentBag<string> TenantIdsSeen { get; private set; } = new();
+
+    public static void Reset() => TenantIdsSeen = new ConcurrentBag<string>();
+}
diff --git a/src/Marten/Events/IEventAppender.cs b/src/Marten/Events/IEventAppender.cs
index 7119b3c03f..b89814f51b 100644
--- a/src/Marten/Events/IEventAppender.cs
+++ b/src/Marten/Events/IEventAppender.cs
@@ -67,10 +67,29 @@ public Task ProcessEventsAsync(EventGraph eventGraph, DocumentSessionBase sessio
         var streamActions = session.WorkTracker.Streams.Where(Matches).ToArray();
         foreach (var action in streamActions)
         {
+            // Route the stream (and its mt_events rows) to the default tenant so
+            // that AddGlobalProjection's documented "single-tenanted within a
+            // conjoined tenancy" guarantee still holds.
+            //
+            // NOTE: StreamAction.TenantId's setter propagates the new value onto
+            // every IEvent in the stream (see JasperFx.Events StreamAction). If
+            // we stopped there, inline SingleStreamProjection Create(IEvent<T>)
+            // / Apply(IEvent<T>, TDoc) convention methods would read
+            // @event.TenantId as "*DEFAULT*" instead of the session tenant
+            // under which the user appended them — see
+            // https://github.com/JasperFx/marten/issues/4270. To preserve the
+            // session tenant on in-flight events, we capture the original tenant
+            // before forcing the default on the action, then restore it on each
+            // event. The stream-level action.TenantId keeps the default tenant,
+            // so storage (mt_events / mt_streams) stays single-tenanted.
+            var originalTenantId = action.TenantId;
             action.TenantId = StorageConstants.DefaultTenantId;
-            foreach (var e in action.Events)
+            if (originalTenantId != null && originalTenantId != StorageConstants.DefaultTenantId)
             {
-                e.TenantId = StorageConstants.DefaultTenantId;
+                foreach (var e in action.Events)
+                {
+                    e.TenantId = originalTenantId;
+                }
             }
         }