diff --git a/app/Models/Category.php b/app/Models/Category.php
index 9065e713..57dbb38c 100644
--- a/app/Models/Category.php
+++ b/app/Models/Category.php
@@ -19,9 +19,16 @@ class Category extends Model
     public $timestamps = false;
 
     protected $fillable = [
-        'name', 'icon', 'description',
+        'name', 'icon', 'description', 'is_default',
     ];
 
+    protected function casts(): array
+    {
+        return [
+            'is_default' => 'boolean',
+        ];
+    }
+
     public function posts(): HasMany
     {
         return $this->hasMany(Post::class);
diff --git a/database/factories/CategoryFactory.php b/database/factories/CategoryFactory.php
index 28ec0d68..459eb6dc 100644
--- a/database/factories/CategoryFactory.php
+++ b/database/factories/CategoryFactory.php
@@ -15,6 +15,7 @@ public function definition(): array
             'name' => $this->faker->name(),
             'icon' => $this->faker->bothify('##### #####'),
             'description' => $this->faker->sentence(),
+            'is_default' => false,
         ];
     }
 }
diff --git a/database/migrations/2026_07_01_145044_add_is_default_to_categories_table.php b/database/migrations/2026_07_01_145044_add_is_default_to_categories_table.php
new file mode 100644
index 00000000..2e0362e1
--- /dev/null
+++ b/database/migrations/2026_07_01_145044_add_is_default_to_categories_table.php
@@ -0,0 +1,33 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('categories', function (Blueprint $table) {
+            $table->boolean('is_default')->default(false)->after('description')->comment('是否為預設分類');
+        });
+
+        DB::table('categories')
+            ->whereIn('name', ['日常分享', '程式技術', '電玩遊戲'])
+            ->update(['is_default' => true]);
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('categories', function (Blueprint $table) {
+            $table->dropColumn('is_default');
+        });
+    }
+};
diff --git a/tests/Feature/Models/CategoryTest.php b/tests/Feature/Models/CategoryTest.php
index fbe0c4b6..b7dace33 100644
--- a/tests/Feature/Models/CategoryTest.php
+++ b/tests/Feature/Models/CategoryTest.php
@@ -22,3 +22,14 @@
             'name' => $category->name,
         ]));
 })->group('has link with name');
+
+it('marks the seeded categories as default', function () {
+    expect(Category::whereIn('id', [1, 2, 3])->pluck('is_default'))
+        ->each->toBeTrue();
+});
+
+it('defaults is_default to false for new categories', function () {
+    $category = Category::factory()->create();
+
+    expect($category->is_default)->toBeFalse();
+});