diff --git a/compute/src/main/java/org/zstack/compute/zone/ZoneManagerImpl.java b/compute/src/main/java/org/zstack/compute/zone/ZoneManagerImpl.java
index 1537ed30318..e806761229d 100755
--- a/compute/src/main/java/org/zstack/compute/zone/ZoneManagerImpl.java
+++ b/compute/src/main/java/org/zstack/compute/zone/ZoneManagerImpl.java
@@ -152,6 +152,10 @@ private ZoneInventory createZoneFromApiMessage(APICreateZoneMsg msg) {
         new SQLBatch() {
             @Override
             protected void scripts() {
+                if (msg.getDefault() == null && !q(ZoneVO.class).isExists()) {
+                    finalVO.setDefault(true);
+                }
+
                 if (finalVO.isDefault()) {
                     sql(ZoneVO.class)
                             .set(ZoneVO_.isDefault, false)
@@ -181,7 +185,7 @@ private void createDefaultManagementNetworkIpVersionTagIfAbsent(String zoneUuid)
     }
 
     private void createZone(APICreateZoneMsg msg, ReturnValueCompletion<ZoneInventory> completion) {
-        if (msg.getDefault() == null || !msg.getDefault()) {
+        if (msg.getDefault() != null && !msg.getDefault()) {
             completion.success(createZoneFromApiMessage(msg));
             return;
         }
diff --git a/conf/db/upgrade/V5.5.28__schema.sql b/conf/db/upgrade/V5.5.28__schema.sql
index b30064a2a08..cc96f27fdf0 100644
--- a/conf/db/upgrade/V5.5.28__schema.sql
+++ b/conf/db/upgrade/V5.5.28__schema.sql
@@ -199,6 +199,38 @@ WHERE NOT EXISTS (
       AND st.tag LIKE 'managementNetwork::ipVersion::%'
 );
 
+-- ZSTAC-75952: backfill one default zone for deployments affected by zone creation without isDefault.
+DROP PROCEDURE IF EXISTS backfill_default_zone_if_absent;
+DELIMITER $$
+CREATE PROCEDURE backfill_default_zone_if_absent()
+BEGIN
+    DECLARE default_zone_count BIGINT DEFAULT 0;
+    DECLARE first_zone_uuid VARCHAR(32) DEFAULT NULL;
+
+    SELECT COUNT(*) INTO default_zone_count
+    FROM `zstack`.`ZoneVO`
+    WHERE `isDefault` = 1;
+
+    IF default_zone_count = 0 THEN
+        SET first_zone_uuid = (
+            SELECT `uuid`
+            FROM `zstack`.`ZoneVO`
+            ORDER BY `createDate` ASC, `uuid` ASC
+            LIMIT 1
+        );
+
+        IF first_zone_uuid IS NOT NULL THEN
+            UPDATE `zstack`.`ZoneEO`
+            SET `isDefault` = 1
+            WHERE `uuid` = first_zone_uuid;
+        END IF;
+    END IF;
+END $$
+DELIMITER ;
+
+CALL backfill_default_zone_if_absent();
+DROP PROCEDURE IF EXISTS backfill_default_zone_if_absent;
+
 ALTER TABLE `zstack`.`ConsoleProxyAgentVO` ADD COLUMN `consoleProxyOverriddenIpv4` varchar(255) DEFAULT NULL;
 ALTER TABLE `zstack`.`ConsoleProxyAgentVO` ADD COLUMN `consoleProxyOverriddenIpv6` varchar(255) DEFAULT NULL;
 
diff --git a/test/src/test/groovy/org/zstack/test/integration/compute/ZoneCase.groovy b/test/src/test/groovy/org/zstack/test/integration/compute/ZoneCase.groovy
index 67e86160548..c8a864c8c41 100644
--- a/test/src/test/groovy/org/zstack/test/integration/compute/ZoneCase.groovy
+++ b/test/src/test/groovy/org/zstack/test/integration/compute/ZoneCase.groovy
@@ -10,12 +10,13 @@ import org.zstack.testlib.EnvSpec
 import org.zstack.testlib.SubCase
 
 /**
- * 1. test create zone as default zone
- * 2. test batch create zone as default zone
- * 3. test update zone to default zone
- * 4. test batch update zone to default zone
- * 5. test query default zone
- * 6. test delete default zone
+ * 1. test first created zone defaults to default zone
+ * 2. test create zone as default zone
+ * 3. test batch create zone as default zone
+ * 4. test update zone to default zone
+ * 5. test batch update zone to default zone
+ * 6. test query default zone
+ * 7. test delete default zone
  */
 class ZoneCase extends SubCase {
     EnvSpec env
@@ -33,6 +34,7 @@ class ZoneCase extends SubCase {
     @Override
     void test() {
         env.create {
+            testFirstCreatedZoneDefaultsToDefaultZone()
             testCreateZoneAsDefaultZone()
             testBatchCreateZoneAsDefaultZone()
             testUpdateZoneToDefaultZone()
@@ -48,6 +50,22 @@ class ZoneCase extends SubCase {
         env.delete()
     }
 
+    void testFirstCreatedZoneDefaultsToDefaultZone() {
+        ZoneInventory zone = createZone {
+            name = "first-zone"
+            description = "first-zone"
+        } as ZoneInventory
+
+        assert zone.isDefault
+        assert Q.New(ZoneVO.class)
+                .eq(ZoneVO_.uuid, zone.uuid)
+                .eq(ZoneVO_.isDefault, true)
+                .isExists()
+        assert Q.New(ZoneVO.class)
+                .eq(ZoneVO_.isDefault, true)
+                .count() == 1
+    }
+
     void testCreateZoneAsDefaultZone() {
         ZoneInventory zone = createZone {
             name = "zone1"