diff --git a/lib/model.js b/lib/model.js
index 51d33bf5a41..a146eea1ef0 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -36,6 +36,7 @@ const assignVals = require('./helpers/populate/assignVals');
 const castBulkWrite = require('./helpers/model/castBulkWrite');
 const createPopulateQueryFilter = require('./helpers/populate/createPopulateQueryFilter');
 const getDefaultBulkwriteResult = require('./helpers/getDefaultBulkwriteResult');
+const getSchemaDiscriminatorByValue = require('./helpers/discriminator/getSchemaDiscriminatorByValue');
 const discriminator = require('./helpers/model/discriminator');
 const firstKey = require('./helpers/firstKey');
 const each = require('./helpers/each');
@@ -4474,7 +4475,11 @@ Model.validate = function validate(obj, pathsToValidate, context, callback) {
   }
 
   return this.db.base._promiseOrCallback(callback, cb => {
-    const schema = this.schema;
+    let schema = this.schema;
+    const discriminatorKey = schema.options.discriminatorKey;
+    if (schema.discriminators != null && obj != null && obj[discriminatorKey] != null) {
+      schema = getSchemaDiscriminatorByValue(schema, obj[discriminatorKey]) || schema;
+    }
     let paths = Object.keys(schema.paths);
 
     if (pathsToValidate != null) {
diff --git a/test/model.test.js b/test/model.test.js
index 5728e286114..069a5f64502 100644
--- a/test/model.test.js
+++ b/test/model.test.js
@@ -7458,119 +7458,6 @@ describe('Model', function() {
     });
   });
 
-  it('Model.validate() (gh-7587)', async function() {
-    const Model = db.model('Test', new Schema({
-      name: {
-        first: {
-          type: String,
-          required: true
-        },
-        last: {
-          type: String,
-          required: true
-        }
-      },
-      age: {
-        type: Number,
-        required: true
-      },
-      comments: [{ name: { type: String, required: true } }]
-    }));
-
-
-    let err = null;
-    let obj = null;
-
-    err = await Model.validate({ age: null }, ['age']).
-      then(() => null, err => err);
-    assert.ok(err);
-    assert.deepEqual(Object.keys(err.errors), ['age']);
-
-    err = await Model.validate({ name: {} }, ['name']).
-      then(() => null, err => err);
-    assert.ok(err);
-    assert.deepEqual(Object.keys(err.errors), ['name.first', 'name.last']);
-
-    obj = { name: { first: 'foo' } };
-    err = await Model.validate(obj, ['name']).
-      then(() => null, err => err);
-    assert.ok(err);
-    assert.deepEqual(Object.keys(err.errors), ['name.last']);
-
-    obj = { comments: [{ name: 'test' }, {}] };
-    err = await Model.validate(obj, ['comments']).
-      then(() => null, err => err);
-    assert.ok(err);
-    assert.deepEqual(Object.keys(err.errors), ['comments.name']);
-
-    obj = { age: '42' };
-    await Model.validate(obj, ['age']);
-    assert.strictEqual(obj.age, 42);
-  });
-
-  it('Model.validate(...) validates paths in arrays (gh-8821)', async function() {
-    const userSchema = new Schema({
-      friends: [{ type: String, required: true, minlength: 3 }]
-    });
-
-    const User = db.model('User', userSchema);
-
-    const err = await User.validate({ friends: [null, 'A'] }).catch(err => err);
-
-    assert.ok(err.errors['friends.0']);
-    assert.ok(err.errors['friends.1']);
-
-  });
-
-  it('Model.validate() works with arrays (gh-10669)', async function() {
-    const testSchema = new Schema({
-      docs: [String]
-    });
-
-    const Test = db.model('Test', testSchema);
-
-    const test = { docs: ['6132655f2cdb9d94eaebc09b'] };
-
-    const err = await Test.validate(test);
-    assert.ifError(err);
-  });
-
-  it('Model.validate(...) uses document instance as context by default (gh-10132)', async function() {
-    const userSchema = new Schema({
-      name: {
-        type: String,
-        required: function() {
-          return this.nameRequired;
-        }
-      },
-      nameRequired: Boolean
-    });
-
-    const User = db.model('User', userSchema);
-
-    const user = new User({ name: 'test', nameRequired: false });
-    const err = await User.validate(user).catch(err => err);
-
-    assert.ifError(err);
-
-  });
-  it('Model.validate(...) uses object as context by default (gh-10346)', async() => {
-
-    const userSchema = new mongoose.Schema({
-      name: { type: String, required: true },
-      age: { type: Number, required() {return this && this.name === 'John';} }
-    });
-
-    const User = db.model('User', userSchema);
-
-    const err1 = await User.validate({ name: 'John' }).then(() => null, err => err);
-    assert.ok(err1);
-
-    const err2 = await User.validate({ name: 'Sam' }).then(() => null, err => err);
-    assert.ok(err2 === null);
-
-  });
-
   it('sets correct `Document#op` with `save()` (gh-8439)', function() {
     const schema = Schema({ name: String });
     const ops = [];
diff --git a/test/model.validate.test.js b/test/model.validate.test.js
new file mode 100644
index 00000000000..30891867ff3
--- /dev/null
+++ b/test/model.validate.test.js
@@ -0,0 +1,147 @@
+'use strict';
+
+const start = require('./common');
+
+const assert = require('assert');
+
+const mongoose = start.mongoose;
+const Schema = mongoose.Schema;
+
+describe('model: validate: ', function() {
+  beforeEach(() => mongoose.deleteModel(/.*/));
+  after(() => mongoose.deleteModel(/.*/));
+
+  it('Model.validate() (gh-7587)', async function() {
+    const Model = mongoose.model('Test', new Schema({
+      name: {
+        first: {
+          type: String,
+          required: true
+        },
+        last: {
+          type: String,
+          required: true
+        }
+      },
+      age: {
+        type: Number,
+        required: true
+      },
+      comments: [{ name: { type: String, required: true } }]
+    }));
+
+
+    let err = null;
+    let obj = null;
+
+    err = await Model.validate({ age: null }, ['age']).
+      then(() => null, err => err);
+    assert.ok(err);
+    assert.deepEqual(Object.keys(err.errors), ['age']);
+
+    err = await Model.validate({ name: {} }, ['name']).
+      then(() => null, err => err);
+    assert.ok(err);
+    assert.deepEqual(Object.keys(err.errors), ['name.first', 'name.last']);
+
+    obj = { name: { first: 'foo' } };
+    err = await Model.validate(obj, ['name']).
+      then(() => null, err => err);
+    assert.ok(err);
+    assert.deepEqual(Object.keys(err.errors), ['name.last']);
+
+    obj = { comments: [{ name: 'test' }, {}] };
+    err = await Model.validate(obj, ['comments']).
+      then(() => null, err => err);
+    assert.ok(err);
+    assert.deepEqual(Object.keys(err.errors), ['comments.name']);
+
+    obj = { age: '42' };
+    await Model.validate(obj, ['age']);
+    assert.strictEqual(obj.age, 42);
+  });
+
+  it('Model.validate(...) validates paths in arrays (gh-8821)', async function() {
+    const userSchema = new Schema({
+      friends: [{ type: String, required: true, minlength: 3 }]
+    });
+
+    const User = mongoose.model('User', userSchema);
+
+    const err = await User.validate({ friends: [null, 'A'] }).catch(err => err);
+
+    assert.ok(err.errors['friends.0']);
+    assert.ok(err.errors['friends.1']);
+  });
+
+  it('Model.validate(...) respects discriminators (gh-12621)', async function() {
+    const CatSchema = new Schema({ meows: { type: Boolean, required: true } });
+    const DogSchema = new Schema({ barks: { type: Boolean, required: true } });
+    const AnimalSchema = new Schema(
+      { id: String },
+      { discriminatorKey: 'kind' }
+    );
+    AnimalSchema.discriminator('cat', CatSchema);
+    AnimalSchema.discriminator('dog', DogSchema);
+
+    const Animal = mongoose.model('Test', AnimalSchema);
+
+    const invalidPet1 = new Animal({
+      id: '123',
+      kind: 'dog',
+      meows: true
+    });
+
+    const err = await Animal.validate(invalidPet1).then(() => null, err => err);
+    assert.ok(err);
+    assert.ok(err.errors['barks']);
+  });
+
+  it('Model.validate() works with arrays (gh-10669)', async function() {
+    const testSchema = new Schema({
+      docs: [String]
+    });
+
+    const Test = mongoose.model('Test', testSchema);
+
+    const test = { docs: ['6132655f2cdb9d94eaebc09b'] };
+
+    const err = await Test.validate(test);
+    assert.ifError(err);
+  });
+
+  it('Model.validate(...) uses document instance as context by default (gh-10132)', async function() {
+    const userSchema = new Schema({
+      name: {
+        type: String,
+        required: function() {
+          return this.nameRequired;
+        }
+      },
+      nameRequired: Boolean
+    });
+
+    const User = mongoose.model('User', userSchema);
+
+    const user = new User({ name: 'test', nameRequired: false });
+    const err = await User.validate(user).catch(err => err);
+
+    assert.ifError(err);
+
+  });
+  it('Model.validate(...) uses object as context by default (gh-10346)', async() => {
+
+    const userSchema = new mongoose.Schema({
+      name: { type: String, required: true },
+      age: { type: Number, required() {return this && this.name === 'John';} }
+    });
+
+    const User = mongoose.model('User', userSchema);
+
+    const err1 = await User.validate({ name: 'John' }).then(() => null, err => err);
+    assert.ok(err1);
+
+    const err2 = await User.validate({ name: 'Sam' }).then(() => null, err => err);
+    assert.ok(err2 === null);
+  });
+});