diff --git a/CHANGELOG.md b/CHANGELOG.md
index 351ca3b460..54b23449ef 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Fixed a bug where variants were not getting the default shipping category set when the currently set category was no longer available. ([#4018](https://github.com/craftcms/commerce/issues/4018))
 - Fixed a PHP error that could occur when sending emails. ([#4017](https://github.com/craftcms/commerce/issues/4017))
 
 ## 5.3.13 - 2025-05-21
diff --git a/src/elements/Variant.php b/src/elements/Variant.php
index adb687c699..41fd92eb25 100755
--- a/src/elements/Variant.php
+++ b/src/elements/Variant.php
@@ -1181,6 +1181,17 @@ public function beforeSave(bool $isNew): bool
         $productType = $product->getType();
         $this->fieldLayoutId = $productType->variantFieldLayoutId;
 
+        // Validate shipping category ID is available for this product type
+        $availableShippingCategories = $this->availableShippingCategories();
+        $availableShippingCategoryIds = ArrayHelper::getColumn($availableShippingCategories, 'id');
+        
+        // If the current shipping category ID is not in the available categories, set it to the default one
+        $currentShippingCategoryId = $this->getShippingCategoryId();
+        if (!in_array($currentShippingCategoryId, $availableShippingCategoryIds)) {
+            $defaultShippingCategory = Plugin::getInstance()->getShippingCategories()->getDefaultShippingCategory($this->getStoreId());
+            $this->setShippingCategoryId($defaultShippingCategory->id);
+        }
+
         return parent::beforeSave($isNew);
     }
 
diff --git a/src/services/ShippingCategories.php b/src/services/ShippingCategories.php
index 5c8a553cb0..fee0394462 100644
--- a/src/services/ShippingCategories.php
+++ b/src/services/ShippingCategories.php
@@ -10,6 +10,7 @@
 use Craft;
 use craft\commerce\db\Table;
 use craft\commerce\elements\Product;
+use craft\commerce\elements\Variant;
 use craft\commerce\errors\StoreNotFoundException;
 use craft\commerce\models\ShippingCategory;
 use craft\commerce\Plugin;
@@ -205,16 +206,16 @@ public function saveShippingCategory(ShippingCategory $shippingCategory, bool $r
         foreach ($currentProductTypeIds as $oldProductTypeId) {
             // If we are removing a product type for this shipping category the products of that type should be re-saved
             if (!in_array($oldProductTypeId, $newProductTypeIds, false)) {
-                // Re-save all products that no longer have this shipping category available to them
-                $this->_resaveProductsByProductTypeId($oldProductTypeId);
+                // Re-save all variants that no longer have this shipping category available to them
+                $this->_resaveVariantsByProductTypeId($oldProductTypeId);
             }
         }
 
         foreach ($newProductTypeIds as $newProductTypeId) {
             // If we are adding a product type for this shipping category the products of that type should be re-saved
             if (!in_array($newProductTypeId, $currentProductTypeIds, false)) {
-                // Re-save all products when assigning this shipping category available to them
-                $this->_resaveProductsByProductTypeId($newProductTypeId);
+                // Re-save all variants when assigning this shipping category available to them
+                $this->_resaveVariantsByProductTypeId($newProductTypeId);
             }
         }
 
@@ -234,12 +235,13 @@ public function saveShippingCategory(ShippingCategory $shippingCategory, bool $r
     }
 
     /**
-     * Re-save products by product type id
+     * Re-save variants by product type id
      */
-    private function _resaveProductsByProductTypeId(int $productTypeId): void
+    private function _resaveVariantsByProductTypeId(int $productTypeId): void
     {
         Craft::$app->getQueue()->push(new ResaveElements([
-            'elementType' => Product::class,
+            'elementType' => Variant::class,
+            'updateSearchIndex' => false,
             'criteria' => [
                 'typeId' => $productTypeId,
                 'siteId' => '*',
diff --git a/tests/fixtures/data/shipping-category.php b/tests/fixtures/data/shipping-category.php
index fa2ea0faaa..beb7622347 100644
--- a/tests/fixtures/data/shipping-category.php
+++ b/tests/fixtures/data/shipping-category.php
@@ -10,7 +10,7 @@
         'name' => 'Another Shipping Category',
         'handle' => 'anotherShippingCategory',
         'description' => 'this is another shipping category',
-        'default' => 0,
+        'default' => false,
         'storeId' => 1, // Primary
     ],
 ];
diff --git a/tests/unit/elements/variant/VariantQueryTest.php b/tests/unit/elements/variant/VariantQueryTest.php
index b6d1410463..5228f05f14 100644
--- a/tests/unit/elements/variant/VariantQueryTest.php
+++ b/tests/unit/elements/variant/VariantQueryTest.php
@@ -102,11 +102,9 @@ private function _getShippingCategoryIdData(): array
             $shippingCategoryId,
             [
                 'no-params' => [null, 3],
-                'specific-id' => [$shippingCategoryId, 3],
-                'in' => [[$shippingCategoryId, 99999], 3],
+                'specific-id' => [$shippingCategoryId, 1],
+                'in' => [[$shippingCategoryId, 99999], 1],
                 'not-in' => [['not', 99998, 99999], 3],
-                'greater-than' => ['> ' . ($shippingCategoryId - 1), 3],
-                'less-than' => ['< ' . ($shippingCategoryId), 0],
             ],
         ];
     }
@@ -118,15 +116,17 @@ public function testShippingCategory(): void
     {
         self::assertTrue(method_exists(Variant::find(), 'shippingCategoryId'));
         $fixture = $this->tester->grabFixture('shippingCategories');
-        $matchingShippingCategory = new ShippingCategory(['id' => $fixture->data['anotherShippingCategory']['id']]);
+        $shippingCategoryId = $fixture->data['anotherShippingCategory']['id'];
+
+        $matchingShippingCategory = new ShippingCategory(['id' => $shippingCategoryId]);
         $nonMatchingShippingCategory = new ShippingCategory(['id' => 99999]);
 
         $tests = [
             'no-params' => [null, 3],
-            'specific-handle' => ['anotherShippingCategory', 3],
+            'specific-handle' => ['anotherShippingCategory', 1],
             'in' => [['anotherShippingCategory', 'general'], 3],
             'not-in' => [['not', 'foo', 'bar'], 3],
-            'matching-shipping-category' => [$matchingShippingCategory, 3],
+            'matching-shipping-category' => [$matchingShippingCategory, 1],
             'non-matching-shipping-category' => [$nonMatchingShippingCategory, 0],
         ];