diff --git a/lib/document.js b/lib/document.js
index 52f39f4328..55b916893e 100644
--- a/lib/document.js
+++ b/lib/document.js
@@ -4256,24 +4256,25 @@ function applySchemaTypeTransforms(self, json) {
 
   for (const path of paths) {
     const schematype = schema.paths[path];
-    if (typeof schematype.options.transform === 'function') {
+    const topLevelTransformFunction = schematype.options.transform ?? schematype.constructor?.defaultOptions?.transform;
+    const embeddedSchemaTypeTransformFunction = schematype.$embeddedSchemaType?.options?.transform
+      ?? schematype.$embeddedSchemaType?.constructor?.defaultOptions?.transform;
+    if (typeof topLevelTransformFunction === 'function') {
       const val = self.$get(path);
       if (val === undefined) {
         continue;
       }
-      const transformedValue = schematype.options.transform.call(self, val);
+      const transformedValue = topLevelTransformFunction.call(self, val);
       throwErrorIfPromise(path, transformedValue);
       utils.setValue(path, transformedValue, json);
-    } else if (schematype.$embeddedSchemaType != null &&
-        typeof schematype.$embeddedSchemaType.options.transform === 'function') {
+    } else if (typeof embeddedSchemaTypeTransformFunction === 'function') {
       const val = self.$get(path);
       if (val === undefined) {
         continue;
       }
       const vals = [].concat(val);
-      const transform = schematype.$embeddedSchemaType.options.transform;
       for (let i = 0; i < vals.length; ++i) {
-        const transformedValue = transform.call(self, vals[i]);
+        const transformedValue = embeddedSchemaTypeTransformFunction.call(self, vals[i]);
         vals[i] = transformedValue;
         throwErrorIfPromise(path, transformedValue);
       }
diff --git a/package.json b/package.json
index da67323104..1f11693d60 100644
--- a/package.json
+++ b/package.json
@@ -30,8 +30,8 @@
   "devDependencies": {
     "@babel/core": "7.26.0",
     "@babel/preset-env": "7.26.0",
-    "@typescript-eslint/eslint-plugin": "^8.18.0",
-    "@typescript-eslint/parser": "^8.18.0",
+    "@typescript-eslint/eslint-plugin": "8.18.0",
+    "@typescript-eslint/parser": "8.18.0",
     "acquit": "1.3.0",
     "acquit-ignore": "0.2.1",
     "acquit-require": "0.1.1",
diff --git a/test/document.test.js b/test/document.test.js
index 4317795223..e0889ec1d2 100644
--- a/test/document.test.js
+++ b/test/document.test.js
@@ -14225,6 +14225,46 @@ describe('document', function() {
     assert.strictEqual(duplicateKeyError.message, 'Email must be unique');
     assert.strictEqual(duplicateKeyError.cause.code, 11000);
   });
+
+  it('supports global transforms per schematype (gh-15084)', async function() {
+    class SchemaCustomType extends mongoose.SchemaType {
+      constructor(key, options) {
+        super(key, options, 'CustomType');
+      }
+
+      cast(value) {
+        if (value === null) return null;
+        return new CustomType(value);
+      }
+    }
+    SchemaCustomType.schemaName = 'CustomType';
+
+    class CustomType {
+      constructor(value) {
+        this.value = value;
+      }
+    }
+
+    mongoose.Schema.Types.CustomType = SchemaCustomType;
+
+    const Model = db.model(
+      'Test',
+      new mongoose.Schema({
+        value: { type: mongoose.Schema.Types.CustomType }
+      })
+    );
+
+    const _id = new mongoose.Types.ObjectId('0'.repeat(24));
+    const doc = new Model({ _id });
+    doc.value = 1;
+
+    mongoose.Schema.Types.CustomType.set('transform', v => v == null ? v : v.value);
+
+    assert.deepStrictEqual(doc.toJSON(), { _id, value: 1 });
+    assert.deepStrictEqual(doc.toObject(), { _id, value: 1 });
+
+    delete mongoose.Schema.Types.CustomType;
+  });
 });
 
 describe('Check if instance function that is supplied in schema option is available', function() {