diff --git a/CHANGES.txt b/CHANGES.txt
index b4a5206aa326..23a694b0bd0d 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,4 +1,5 @@
 5.1
+ * Make JsonUtils serialize Instant always with the same format (CASSANDRA-20209)
  * Port Harry v2 to trunk (CASSANDRA-20200)
  * Enable filtering of snapshots on keyspace, table and snapshot name in nodetool listsnapshots (CASSANDRA-20151)
  * Create manifest upon loading where it does not exist or enrich it (CASSANDRA-20150)
diff --git a/src/java/org/apache/cassandra/utils/JsonUtils.java b/src/java/org/apache/cassandra/utils/JsonUtils.java
index aa0963bf5859..4868fedd4598 100644
--- a/src/java/org/apache/cassandra/utils/JsonUtils.java
+++ b/src/java/org/apache/cassandra/utils/JsonUtils.java
@@ -21,6 +21,9 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.time.Instant;
+import java.time.ZoneOffset;
+import java.time.format.DateTimeFormatter;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
@@ -29,8 +32,8 @@
 import com.fasterxml.jackson.core.util.BufferRecyclers;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.ObjectWriter;
-import com.fasterxml.jackson.databind.SerializationFeature;
 import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
+import com.fasterxml.jackson.datatype.jsr310.ser.InstantSerializer;
 import org.apache.cassandra.io.util.File;
 import org.apache.cassandra.io.util.FileInputStreamPlus;
 import org.apache.cassandra.io.util.FileOutputStreamPlus;
@@ -44,10 +47,20 @@ public final class JsonUtils
     public static final ObjectMapper JSON_OBJECT_MAPPER = new ObjectMapper(new JsonFactory()); // checkstyle: permit this instantiation
     public static final ObjectWriter JSON_OBJECT_PRETTY_WRITER;
 
+    private static class GlobalInstantSerializer extends InstantSerializer
+    {
+        private GlobalInstantSerializer()
+        {
+            super(InstantSerializer.INSTANCE,
+                  false,
+                  false,
+                  DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'").withZone(ZoneOffset.UTC));
+        }
+    }
+
     static
     {
-        JSON_OBJECT_MAPPER.registerModule(new JavaTimeModule());
-        JSON_OBJECT_MAPPER.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
+        JSON_OBJECT_MAPPER.registerModule(new JavaTimeModule().addSerializer(Instant.class, new GlobalInstantSerializer()));
         JSON_OBJECT_PRETTY_WRITER = JSON_OBJECT_MAPPER.writerWithDefaultPrettyPrinter();
     }
 
diff --git a/test/unit/org/apache/cassandra/db/ColumnFamilyStoreTest.java b/test/unit/org/apache/cassandra/db/ColumnFamilyStoreTest.java
index 7f5b28c7596d..0febbe660cfa 100644
--- a/test/unit/org/apache/cassandra/db/ColumnFamilyStoreTest.java
+++ b/test/unit/org/apache/cassandra/db/ColumnFamilyStoreTest.java
@@ -449,9 +449,8 @@ private void testTrueSnapshotSizeInternal(String keyspace, String table, boolean
         long anotherWithIndexSize = listedSnapshots.get("snapshot2_with_index").computeSizeOnDiskBytes();
         long anotherWithIndexTrueSize = listedSnapshots.get("snapshot2_with_index").computeTrueSizeBytes();
 
-        // TODO CASSANDRA-20209
-        assertTrue(withIndexSize == anotherWithIndexSize || (withIndexSize + 4 == anotherWithIndexSize) || (withIndexSize - 4 == anotherWithIndexSize));
-        assertTrue(withIndexTrueSize == anotherWithIndexTrueSize || (withIndexTrueSize + 4 == anotherWithIndexTrueSize) || (withIndexTrueSize - 4 == anotherWithIndexTrueSize));
+        assertEquals(withIndexSize, anotherWithIndexSize);
+        assertEquals(withIndexTrueSize, anotherWithIndexTrueSize);
     }
 
     private void rebuildIndices(ColumnFamilyStore cfs)
diff --git a/test/unit/org/apache/cassandra/utils/JsonUtilsTest.java b/test/unit/org/apache/cassandra/utils/JsonUtilsTest.java
new file mode 100644
index 000000000000..1bd32079f8f4
--- /dev/null
+++ b/test/unit/org/apache/cassandra/utils/JsonUtilsTest.java
@@ -0,0 +1,36 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.utils;
+
+import org.junit.Test;
+
+import static java.time.Instant.parse;
+import static org.apache.cassandra.utils.JsonUtils.writeAsJsonString;
+import static org.junit.Assert.assertEquals;
+
+public class JsonUtilsTest
+{
+    @Test
+    public void testTimestampSerialisation()
+    {
+        assertEquals("\"2025-01-15T13:26:45.040Z\"", writeAsJsonString(parse("2025-01-15T13:26:45.04Z")));
+        assertEquals("\"2025-01-15T13:26:45.100Z\"", writeAsJsonString(parse("2025-01-15T13:26:45.1Z")));
+        assertEquals("\"2025-01-15T13:26:45.123Z\"", writeAsJsonString(parse("2025-01-15T13:26:45.123Z")));
+    }
+}