diff --git a/src/NATS.Server/JetStream/InterestRetentionPolicy.cs b/src/NATS.Server/JetStream/InterestRetentionPolicy.cs
new file mode 100644
index 0000000..dabad26
--- /dev/null
+++ b/src/NATS.Server/JetStream/InterestRetentionPolicy.cs
@@ -0,0 +1,75 @@
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.JetStream;
+
+/// <summary>
+/// Tracks per-consumer interest and determines when messages can be removed
+/// under Interest retention policy. A message should be retained until all
+/// interested consumers have acknowledged it.
+/// Go reference: stream.go checkInterestState/noInterest.
+/// </summary>
+public sealed class InterestRetentionPolicy
+{
+    // consumer → filter subject pattern
+    private readonly Dictionary<string, string> _interests = new(StringComparer.Ordinal);
+    // seq → set of consumers that have acked this sequence
+    private readonly Dictionary<ulong, HashSet<string>> _acks = new();
+
+    /// <summary>
+    /// Register a consumer's interest in a subject pattern.
+    /// </summary>
+    public void RegisterInterest(string consumer, string filterSubject)
+    {
+        _interests[consumer] = filterSubject;
+    }
+
+    /// <summary>
+    /// Remove a consumer's interest (e.g., on deletion).
+    /// </summary>
+    public void UnregisterInterest(string consumer)
+    {
+        _interests.Remove(consumer);
+    }
+
+    /// <summary>
+    /// Record that a consumer has acknowledged delivery of a sequence.
+    /// </summary>
+    public void AcknowledgeDelivery(string consumer, ulong seq)
+    {
+        if (!_acks.TryGetValue(seq, out var ackedBy))
+        {
+            ackedBy = new HashSet<string>(StringComparer.Ordinal);
+            _acks[seq] = ackedBy;
+        }
+        ackedBy.Add(consumer);
+    }
+
+    /// <summary>
+    /// Returns true if the message should be retained (i.e., at least one
+    /// interested consumer has NOT yet acknowledged it).
+    /// A consumer is "interested" if its filter subject matches the message subject.
+    /// </summary>
+    public bool ShouldRetain(ulong seq, string msgSubject)
+    {
+        _acks.TryGetValue(seq, out var ackedBy);
+
+        foreach (var (consumer, filterSubject) in _interests)
+        {
+            // Check if this consumer is interested in this message's subject
+            if (!SubjectMatch.MatchLiteral(msgSubject, filterSubject))
+                continue;
+
+            // Consumer is interested — has it acked?
+            if (ackedBy == null || !ackedBy.Contains(consumer))
+                return true; // Not yet acked — must retain
+        }
+
+        // All interested consumers have acked (or no one is interested)
+        // Clean up ack tracking for this sequence
+        _acks.Remove(seq);
+        return false;
+    }
+
+    /// <summary>Number of registered consumers.</summary>
+    public int ConsumerCount => _interests.Count;
+}
diff --git a/tests/NATS.Server.Tests/JetStream/InterestRetentionTests.cs b/tests/NATS.Server.Tests/JetStream/InterestRetentionTests.cs
new file mode 100644
index 0000000..3b39596
--- /dev/null
+++ b/tests/NATS.Server.Tests/JetStream/InterestRetentionTests.cs
@@ -0,0 +1,91 @@
+using NATS.Server.JetStream;
+
+namespace NATS.Server.Tests.JetStream;
+
+/// <summary>
+/// Tests for InterestRetentionPolicy per-consumer ack tracking.
+/// Go reference: stream.go checkInterestState/noInterest.
+/// </summary>
+public class InterestRetentionTests
+{
+    [Fact]
+    public void ShouldRetain_true_when_consumers_have_not_acked()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.RegisterInterest("consumer-A", "orders.>");
+        policy.RegisterInterest("consumer-B", "orders.>");
+
+        policy.ShouldRetain(1, "orders.new").ShouldBeTrue();
+    }
+
+    [Fact]
+    public void ShouldRetain_false_when_all_consumers_acked()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.RegisterInterest("consumer-A", "orders.>");
+        policy.RegisterInterest("consumer-B", "orders.>");
+
+        policy.AcknowledgeDelivery("consumer-A", 1);
+        policy.ShouldRetain(1, "orders.new").ShouldBeTrue(); // B hasn't acked
+
+        policy.AcknowledgeDelivery("consumer-B", 1);
+        policy.ShouldRetain(1, "orders.new").ShouldBeFalse(); // both acked
+    }
+
+    [Fact]
+    public void ShouldRetain_ignores_consumers_without_interest()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.RegisterInterest("consumer-A", "orders.>");
+        policy.RegisterInterest("consumer-B", "billing.>"); // no interest in orders
+
+        policy.AcknowledgeDelivery("consumer-A", 1);
+        policy.ShouldRetain(1, "orders.new").ShouldBeFalse(); // B has no interest
+    }
+
+    [Fact]
+    public void UnregisterInterest_removes_consumer()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.RegisterInterest("consumer-A", "x.>");
+        policy.RegisterInterest("consumer-B", "x.>");
+
+        policy.UnregisterInterest("consumer-B");
+
+        // Only A needs to ack
+        policy.AcknowledgeDelivery("consumer-A", 1);
+        policy.ShouldRetain(1, "x.y").ShouldBeFalse();
+    }
+
+    [Fact]
+    public void ShouldRetain_false_when_no_consumers_registered()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.ShouldRetain(1, "any.subject").ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Multiple_sequences_tracked_independently()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.RegisterInterest("c1", "x.>");
+
+        policy.AcknowledgeDelivery("c1", 1);
+        policy.ShouldRetain(1, "x.y").ShouldBeFalse();
+        policy.ShouldRetain(2, "x.y").ShouldBeTrue(); // seq 2 not acked
+    }
+
+    [Fact]
+    public void ConsumerCount_tracks_registrations()
+    {
+        var policy = new InterestRetentionPolicy();
+        policy.ConsumerCount.ShouldBe(0);
+
+        policy.RegisterInterest("c1", "x.>");
+        policy.RegisterInterest("c2", "y.>");
+        policy.ConsumerCount.ShouldBe(2);
+
+        policy.UnregisterInterest("c1");
+        policy.ConsumerCount.ShouldBe(1);
+    }
+}