diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/Mapping/AggregateModeMapper.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/Mapping/AggregateModeMapper.cs
new file mode 100644
index 00000000..f3d8b3a0
--- /dev/null
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/Mapping/AggregateModeMapper.cs
@@ -0,0 +1,39 @@
+using ZB.MOM.WW.HistorianGateway.Contracts.Grpc;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Mapping;
+
+/// <summary>
+/// Maps the driver-agnostic <see cref="HistoryAggregateType"/> (OPC UA Part 13 aggregate) onto the
+/// gateway's native <see cref="RetrievalMode"/>.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Average/Minimum/Maximum line up with the legacy Wonderware client's aggregate mapping. The two
+/// remaining members are now served by <b>native</b> gateway retrieval modes:
+/// <see cref="HistoryAggregateType.Total"/> → <see cref="RetrievalMode.Integral"/> and
+/// <see cref="HistoryAggregateType.Count"/> → <see cref="RetrievalMode.Counter"/>.
+/// </para>
+/// <para>
+/// This replaces the Wonderware-era client-side workarounds (Total derived as Average × interval,
+/// Count approximated from a value count): no client-side scaling is performed any more, so the
+/// gateway path is a strict improvement.
+/// </para>
+/// </remarks>
+internal static class AggregateModeMapper
+{
+    /// <summary>Maps an aggregate function to the gateway retrieval mode.</summary>
+    /// <param name="aggregate">The driver-agnostic aggregate function.</param>
+    /// <returns>The matching gateway <see cref="RetrievalMode"/>.</returns>
+    /// <exception cref="ArgumentOutOfRangeException">A future, unmapped enum member (fails the matrix guard).</exception>
+    public static RetrievalMode ToRetrievalMode(HistoryAggregateType aggregate) => aggregate switch
+    {
+        HistoryAggregateType.Average => RetrievalMode.TimeWeightedAverage,
+        HistoryAggregateType.Minimum => RetrievalMode.MinimumWithTime,
+        HistoryAggregateType.Maximum => RetrievalMode.MaximumWithTime,
+        HistoryAggregateType.Total => RetrievalMode.Integral,
+        HistoryAggregateType.Count => RetrievalMode.Counter,
+        _ => throw new ArgumentOutOfRangeException(
+            nameof(aggregate), aggregate, "Unmapped HistoryAggregateType — add a RetrievalMode mapping."),
+    };
+}
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Mapping/AggregateModeMapperTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Mapping/AggregateModeMapperTests.cs
new file mode 100644
index 00000000..871b513f
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Mapping/AggregateModeMapperTests.cs
@@ -0,0 +1,25 @@
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;        // HistoryAggregateType
+using ZB.MOM.WW.HistorianGateway.Contracts.Grpc;  // RetrievalMode
+using ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Mapping;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests.Mapping;
+
+public sealed class AggregateModeMapperTests
+{
+    [Theory]
+    [InlineData(HistoryAggregateType.Average, RetrievalMode.TimeWeightedAverage)]
+    [InlineData(HistoryAggregateType.Minimum, RetrievalMode.MinimumWithTime)]
+    [InlineData(HistoryAggregateType.Maximum, RetrievalMode.MaximumWithTime)]
+    [InlineData(HistoryAggregateType.Total,   RetrievalMode.Integral)]
+    [InlineData(HistoryAggregateType.Count,   RetrievalMode.Counter)]
+    public void Maps_each_aggregate(HistoryAggregateType a, RetrievalMode expected)
+        => Assert.Equal(expected, AggregateModeMapper.ToRetrievalMode(a));
+
+    [Fact] // matrix guard: a new HistoryAggregateType member must fail here
+    public void Every_aggregate_member_is_mapped()
+    {
+        foreach (var a in Enum.GetValues<HistoryAggregateType>())
+            _ = AggregateModeMapper.ToRetrievalMode(a); // must not throw for any defined member
+    }
+}