diff --git a/lib/helpers/model/castBulkWrite.js b/lib/helpers/model/castBulkWrite.js
index 6420ad7e2e..6d7a780a81 100644
--- a/lib/helpers/model/castBulkWrite.js
+++ b/lib/helpers/model/castBulkWrite.js
@@ -104,7 +104,8 @@ module.exports = function castBulkWrite(originalModel, op, options) {
         op['updateOne']['update'] = castUpdate(model.schema, update, {
           strict: strict,
           upsert: op['updateOne'].upsert,
-          arrayFilters: op['updateOne'].arrayFilters
+          arrayFilters: op['updateOne'].arrayFilters,
+          overwriteDiscriminatorKey: op['updateOne'].overwriteDiscriminatorKey
         }, model, op['updateOne']['filter']);
       } catch (error) {
         return callback(error, null);
@@ -164,7 +165,8 @@ module.exports = function castBulkWrite(originalModel, op, options) {
         op['updateMany']['update'] = castUpdate(model.schema, op['updateMany']['update'], {
           strict: strict,
           upsert: op['updateMany'].upsert,
-          arrayFilters: op['updateMany'].arrayFilters
+          arrayFilters: op['updateMany'].arrayFilters,
+          overwriteDiscriminatorKey: op['updateMany'].overwriteDiscriminatorKey
         }, model, op['updateMany']['filter']);
       } catch (error) {
         return callback(error, null);
diff --git a/lib/helpers/query/castUpdate.js b/lib/helpers/query/castUpdate.js
index d48c592755..3cf30cb0e1 100644
--- a/lib/helpers/query/castUpdate.js
+++ b/lib/helpers/query/castUpdate.js
@@ -8,6 +8,7 @@ const ValidationError = require('../../error/validation');
 const castNumber = require('../../cast/number');
 const cast = require('../../cast');
 const getConstructorName = require('../getConstructorName');
+const getDiscriminatorByValue = require('../discriminator/getDiscriminatorByValue');
 const getEmbeddedDiscriminatorPath = require('./getEmbeddedDiscriminatorPath');
 const handleImmutable = require('./handleImmutable');
 const moveImmutableProperties = require('../update/moveImmutableProperties');
@@ -62,6 +63,27 @@ module.exports = function castUpdate(schema, obj, options, context, filter) {
     return obj;
   }
 
+  if (schema != null &&
+      filter != null &&
+      utils.hasUserDefinedProperty(filter, schema.options.discriminatorKey) &&
+      typeof filter[schema.options.discriminatorKey] !== 'object' &&
+      schema.discriminators != null) {
+    const discriminatorValue = filter[schema.options.discriminatorKey];
+    const byValue = getDiscriminatorByValue(context.model.discriminators, discriminatorValue);
+    schema = schema.discriminators[discriminatorValue] ||
+      (byValue && byValue.schema) ||
+      schema;
+  } else if (schema != null &&
+      options.overwriteDiscriminatorKey &&
+      utils.hasUserDefinedProperty(obj, schema.options.discriminatorKey) &&
+      schema.discriminators != null) {
+    const discriminatorValue = obj[schema.options.discriminatorKey];
+    const byValue = getDiscriminatorByValue(context.model.discriminators, discriminatorValue);
+    schema = schema.discriminators[discriminatorValue] ||
+      (byValue && byValue.schema) ||
+      schema;
+  }
+
   if (options.upsert) {
     moveImmutableProperties(schema, obj, context);
   }
diff --git a/lib/query.js b/lib/query.js
index 32d8b1187a..a45d32fb91 100644
--- a/lib/query.js
+++ b/lib/query.js
@@ -4700,18 +4700,6 @@ Query.prototype._castUpdate = function _castUpdate(obj) {
     upsert = this.options.upsert;
   }
 
-  const filter = this._conditions;
-  if (schema != null &&
-      utils.hasUserDefinedProperty(filter, schema.options.discriminatorKey) &&
-      typeof filter[schema.options.discriminatorKey] !== 'object' &&
-      schema.discriminators != null) {
-    const discriminatorValue = filter[schema.options.discriminatorKey];
-    const byValue = getDiscriminatorByValue(this.model.discriminators, discriminatorValue);
-    schema = schema.discriminators[discriminatorValue] ||
-      (byValue && byValue.schema) ||
-      schema;
-  }
-
   return castUpdate(schema, obj, {
     strict: this._mongooseOptions.strict,
     upsert: upsert,
diff --git a/test/model.test.js b/test/model.test.js
index 5bc1317e52..73d2e809ef 100644
--- a/test/model.test.js
+++ b/test/model.test.js
@@ -4174,6 +4174,51 @@ describe('Model', function() {
         assert.strictEqual(r2.testArray[0].nonexistentProp, undefined);
       });
 
+      it('handles overwriteDiscriminatorKey (gh-15040)', async function() {
+        const dSchema1 = new mongoose.Schema({
+          field1: String
+        });
+        const dSchema2 = new mongoose.Schema({
+          field2: String
+        });
+        const baseSchema = new mongoose.Schema({
+          field: String,
+          key: String
+        }, { discriminatorKey: 'key' });
+        const type1Key = 'Type1';
+        const type2Key = 'Type2';
+
+        baseSchema.discriminator(type1Key, dSchema1);
+        baseSchema.discriminator(type2Key, dSchema2);
+
+        const TestModel = db.model('Test', baseSchema);
+
+        const test = new TestModel({
+          field: 'base field',
+          key: type1Key,
+          field1: 'field1'
+        });
+        const r1 = await test.save();
+        assert.equal(r1.field1, 'field1');
+        assert.equal(r1.key, type1Key);
+
+        const field2 = 'field2';
+        await TestModel.bulkWrite([{
+          updateOne: {
+            filter: { _id: r1._id },
+            update: {
+              key: type2Key,
+              field2
+            },
+            overwriteDiscriminatorKey: true
+          }
+        }]);
+
+        const r2 = await TestModel.findById(r1._id);
+        assert.equal(r2.key, type2Key);
+        assert.equal(r2.field2, field2);
+      });
+
       it('with child timestamps and array filters (gh-7032)', async function() {
         const childSchema = new Schema({ name: String }, { timestamps: true });