diff --git a/sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/CosmosItemSerializerUnitTests.java b/sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/CosmosItemSerializerUnitTests.java
index 82cc7bf9f213..ac866c4c615f 100644
--- a/sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/CosmosItemSerializerUnitTests.java
+++ b/sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/CosmosItemSerializerUnitTests.java
@@ -12,6 +12,7 @@
 
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
+import java.time.Instant;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
@@ -197,4 +198,44 @@ private static class PojoItem
         public String nullValueProperty = null;
         public String emptyValueProperty = "";
     }
+
+    @Test(groups = { "unit" })
+    public void defaultSerializerWritesInstantAsIso8601String() {
+        ObjectMapper mapper = Utils.getDocumentObjectMapper(Configs.getItemSerializationInclusionMode());
+        CosmosItemSerializer serializer = new DefaultCosmosItemSerializer(mapper);
+
+        Instant createdAt = Instant.parse("2023-07-22T09:46:40.123456789Z");
+        PojoItemWithInstant item = new PojoItemWithInstant();
+        item.createdAt = createdAt;
+
+        Map<String, Object> jsonMap = serializer.serialize(item);
+        ByteBuffer buffer = Utils.serializeJsonToByteBuffer(serializer, jsonMap, null, false);
+        String json = new String(buffer.array(), 0, buffer.limit(), StandardCharsets.UTF_8);
+
+        logger.info("Actual: {}", json);
+        assertThat(json).isEqualTo(
+            "{\"id\":\"SomeId\",\"createdAt\":\"" + createdAt + "\"}");
+    }
+
+    @Test(groups = { "unit" })
+    public void defaultSerializerRoundTripsInstant() {
+        ObjectMapper mapper = Utils.getDocumentObjectMapper(Configs.getItemSerializationInclusionMode());
+        CosmosItemSerializer serializer = new DefaultCosmosItemSerializer(mapper);
+
+        Instant createdAt = Instant.parse("2023-07-22T09:46:40.123456789Z");
+        PojoItemWithInstant item = new PojoItemWithInstant();
+        item.createdAt = createdAt;
+
+        Map<String, Object> jsonMap = serializer.serialize(item);
+        PojoItemWithInstant deserialized = serializer.deserialize(jsonMap, PojoItemWithInstant.class);
+
+        assertThat(deserialized.id).isEqualTo(item.id);
+        assertThat(deserialized.createdAt).isEqualTo(createdAt);
+    }
+
+    private static class PojoItemWithInstant
+    {
+        public String id = "SomeId";
+        public Instant createdAt;
+    }
 }
diff --git a/sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/implementation/UtilsTest.java b/sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/implementation/UtilsTest.java
index 0ed8473cd80a..fb9ab7374871 100644
--- a/sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/implementation/UtilsTest.java
+++ b/sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/implementation/UtilsTest.java
@@ -9,9 +9,11 @@
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.SerializationConfig;
+import com.fasterxml.jackson.databind.SerializationFeature;
 import com.fasterxml.jackson.databind.node.ObjectNode;
 import com.fasterxml.jackson.module.afterburner.AfterburnerModule;
 import com.fasterxml.jackson.module.blackbird.BlackbirdModule;
+import java.time.Instant;
 import java.util.List;
 import java.util.stream.Collectors;
 import org.testng.annotations.Test;
@@ -165,4 +167,37 @@ public void serializationInclusionMode() {
             .isEqualTo(
                 JsonInclude.Value.construct(JsonInclude.Include.NON_DEFAULT, JsonInclude.Include.NON_DEFAULT));
     }
+    
+    @Test(groups = { "unit" })
+    public void documentObjectMapperDoesNotWriteDatesAsTimestamps() {
+        assertThat(
+            Utils.getSimpleObjectMapper()
+                .getSerializationConfig()
+                .isEnabled(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS))
+            .isFalse();
+
+        assertThat(
+            Utils.getDocumentObjectMapper(null)
+                .getSerializationConfig()
+                .isEnabled(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS))
+            .isFalse();
+
+        assertThat(
+            Utils.getDocumentObjectMapper("Always")
+                .getSerializationConfig()
+                .isEnabled(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS))
+            .isFalse();
+    }
+
+    @Test(groups = { "unit" })
+    public void instantSerializesAsIso8601String() throws Exception {
+        Instant instant = Instant.parse("2023-07-22T09:46:40.123456789Z");
+
+        String json = Utils.getSimpleObjectMapper().writeValueAsString(instant);
+
+        assertThat(json).isEqualTo("\"" + instant.toString() + "\"");
+
+        Instant roundTripped = Utils.getSimpleObjectMapper().readValue(json, Instant.class);
+        assertThat(roundTripped).isEqualTo(instant);
+    }
 }
diff --git a/sdk/cosmos/azure-cosmos/CHANGELOG.md b/sdk/cosmos/azure-cosmos/CHANGELOG.md
index 7f3b456f7c32..bcce6ef836a7 100644
--- a/sdk/cosmos/azure-cosmos/CHANGELOG.md
+++ b/sdk/cosmos/azure-cosmos/CHANGELOG.md
@@ -5,6 +5,7 @@
 #### Features Added
 
 #### Breaking Changes
+* Fixed `java.time.Instant` (and other `java.time` types handled by `JavaTimeModule`) serializing as a numeric epoch timestamp instead of an ISO-8601 string when using the default `CosmosItemSerializer`. Items previously written with epoch-numeric date fields will now be written with ISO-8601 string values going forward; reading previously-stored epoch-numeric values back into `Instant` fields continues to work unaffected. - See PR [49720](https://github.com/Azure/azure-sdk-for-java/pull/49720).
 
 #### Bugs Fixed
 * Unified request-level consistency override behavior across transports: invalid attempts to upgrade the request consistency level above the account default are now silently ignored instead of returning `BadRequest` in some gateway paths. - See PR [49606](https://github.com/Azure/azure-sdk-for-java/pull/49606).
diff --git a/sdk/cosmos/azure-cosmos/src/main/java/com/azure/cosmos/implementation/Utils.java b/sdk/cosmos/azure-cosmos/src/main/java/com/azure/cosmos/implementation/Utils.java
index a387e72a64ce..21abc9999af0 100644
--- a/sdk/cosmos/azure-cosmos/src/main/java/com/azure/cosmos/implementation/Utils.java
+++ b/sdk/cosmos/azure-cosmos/src/main/java/com/azure/cosmos/implementation/Utils.java
@@ -18,6 +18,7 @@
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.Module;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.SerializationFeature;
 import com.fasterxml.jackson.databind.module.SimpleModule;
 import com.fasterxml.jackson.databind.node.ObjectNode;
 import com.fasterxml.jackson.databind.ser.std.ToStringSerializer;
@@ -152,6 +153,7 @@ private static ObjectMapper createAndInitializeObjectMapper(boolean allowDuplica
         tryToLoadJacksonPerformanceLibrary(objectMapper);
 
         objectMapper.registerModule(new JavaTimeModule());
+        objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
 
         return objectMapper;
     }