diff --git a/README.md b/README.md
index efd0c983e..f2367b760 100644
--- a/README.md
+++ b/README.md
@@ -557,14 +557,35 @@ Thrown on failed validations, with the following properties
 
 ### mixed
 
-Creates a schema that matches all types. All types inherit from this base type
+Creates a schema that matches all types. All types inherit from this base type.
 
-```js
-let schema = yup.mixed();
+```ts
+import { mixed } from 'yup';
 
-schema.isValid(undefined, function (valid) {
-  valid; // => true
-});
+let schema = mixed();
+
+schema.validateSync('string'); // 'string';
+
+schema.validateSync(1); // 1;
+
+schema.validateSync(new Date()); // Date;
+```
+
+Custom types can be implemented by passing a type check function:
+
+```ts
+import { mixed } from 'yup';
+
+let objectIdSchema = yup
+  .mixed((input): input is ObjectId => input instanceof ObjectId)
+  .transform((value: any, input, ctx) => {
+    if (ctx.typeCheck(value)) return value;
+    return new ObjectId(value);
+  });
+
+await objectIdSchema.validate(ObjectId('507f1f77bcf86cd799439011')); // ObjectId("507f1f77bcf86cd799439011")
+
+await objectIdSchema.validate('507f1f77bcf86cd799439011'); // ObjectId("507f1f77bcf86cd799439011")
 ```
 
 #### `mixed.clone(): Schema`
diff --git a/src/array.ts b/src/array.ts
index 1a82a86f9..e9afc96e7 100644
--- a/src/array.ts
+++ b/src/array.ts
@@ -47,7 +47,12 @@ export default class ArraySchema<
   innerType?: ISchema<T, TContext>;
 
   constructor(type?: ISchema<T, TContext>) {
-    super({ type: 'array' });
+    super({
+      type: 'array',
+      check(v: any): v is NonNullable<TIn> {
+        return Array.isArray(v);
+      },
+    });
 
     // `undefined` specifically means uninitialized, as opposed to
     // "no subtype"
@@ -67,10 +72,6 @@ export default class ArraySchema<
     });
   }
 
-  protected _typeCheck(v: any): v is NonNullable<TIn> {
-    return Array.isArray(v);
-  }
-
   private get _subType() {
     return this.innerType;
   }
diff --git a/src/boolean.ts b/src/boolean.ts
index 764dad7b6..ab3448498 100644
--- a/src/boolean.ts
+++ b/src/boolean.ts
@@ -28,7 +28,14 @@ export default class BooleanSchema<
   TFlags extends Flags = '',
 > extends BaseSchema<TType, TContext, TDefault, TFlags> {
   constructor() {
-    super({ type: 'boolean' });
+    super({
+      type: 'boolean',
+      check(v: any): v is NonNullable<TType> {
+        if (v instanceof Boolean) v = v.valueOf();
+
+        return typeof v === 'boolean';
+      },
+    });
 
     this.withMutation(() => {
       this.transform(function (value) {
@@ -41,12 +48,6 @@ export default class BooleanSchema<
     });
   }
 
-  protected _typeCheck(v: any): v is NonNullable<TType> {
-    if (v instanceof Boolean) v = v.valueOf();
-
-    return typeof v === 'boolean';
-  }
-
   isTrue(
     message = locale.isValue,
   ): BooleanSchema<true | Optionals<TType>, TContext, TFlags> {
diff --git a/src/date.ts b/src/date.ts
index c1b068a9f..e2192eaf5 100644
--- a/src/date.ts
+++ b/src/date.ts
@@ -38,7 +38,12 @@ export default class DateSchema<
   static INVALID_DATE = invalidDate;
 
   constructor() {
-    super({ type: 'date' });
+    super({
+      type: 'date',
+      check(v: any): v is NonNullable<TType> {
+        return isDate(v) && !isNaN(v.getTime());
+      },
+    });
 
     this.withMutation(() => {
       this.transform(function (value) {
@@ -52,10 +57,6 @@ export default class DateSchema<
     });
   }
 
-  protected _typeCheck(v: any): v is NonNullable<TType> {
-    return isDate(v) && !isNaN(v.getTime());
-  }
-
   private prepareParam(
     ref: unknown | Ref<Date>,
     name: string,
diff --git a/src/index.ts b/src/index.ts
index e78a0a122..23ef75c08 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,4 +1,8 @@
-import Mixed, { create as mixedCreate, MixedSchema } from './mixed';
+import Mixed, {
+  create as mixedCreate,
+  MixedSchema,
+  MixedOptions,
+} from './mixed';
 import BooleanSchema, { create as boolCreate } from './boolean';
 import StringSchema, { create as stringCreate } from './string';
 import NumberSchema, { create as numberCreate } from './number';
@@ -38,7 +42,13 @@ function addMethod(schemaType: any, name: string, fn: any) {
 
 export type AnyObjectSchema = ObjectSchema<any, any, any, any>;
 
-export type { AnyObject, InferType, InferType as Asserts, AnySchema };
+export type {
+  AnyObject,
+  InferType,
+  InferType as Asserts,
+  AnySchema,
+  MixedOptions,
+};
 
 export {
   mixedCreate as mixed,
diff --git a/src/locale.ts b/src/locale.ts
index d724804ea..25eee1562 100644
--- a/src/locale.ts
+++ b/src/locale.ts
@@ -66,20 +66,24 @@ export interface LocaleObject {
 export let mixed: Required<MixedLocale> = {
   default: '${path} is invalid',
   required: '${path} is a required field',
+  defined: '${path} must be defined',
+  notNull: '${path} cannot be null',
   oneOf: '${path} must be one of the following values: ${values}',
   notOneOf: '${path} must not be one of the following values: ${values}',
   notType: ({ path, type, value, originalValue }) => {
-    let isCast = originalValue != null && originalValue !== value;
-    return (
-      `${path} must be a \`${type}\` type, ` +
-      `but the final value was: \`${printValue(value, true)}\`` +
-      (isCast
+    const castMsg =
+      originalValue != null && originalValue !== value
         ? ` (cast from the value \`${printValue(originalValue, true)}\`).`
-        : '.')
-    );
+        : '.';
+
+    return type !== 'mixed'
+      ? `${path} must be a \`${type}\` type, ` +
+          `but the final value was: \`${printValue(value, true)}\`` +
+          castMsg
+      : `${path} must match the configured type. ` +
+          `The validated value was: \`${printValue(value, true)}\`` +
+          castMsg;
   },
-  defined: '${path} must be defined',
-  notNull: '${path} cannot be null',
 };
 
 export let string: Required<StringLocale> = {
diff --git a/src/mixed.ts b/src/mixed.ts
index aac0e496a..b93f62779 100644
--- a/src/mixed.ts
+++ b/src/mixed.ts
@@ -56,8 +56,17 @@ const Mixed: typeof MixedSchema = BaseSchema as any;
 
 export default Mixed;
 
-export function create<TType = any>() {
-  return new Mixed<TType | undefined>();
+export type TypeGuard<TType> = (value: any) => value is NonNullable<TType>;
+export interface MixedOptions<TType> {
+  type?: string;
+  check?: TypeGuard<TType>;
+}
+export function create<TType = any>(
+  spec?: MixedOptions<TType> | TypeGuard<TType>,
+) {
+  return new Mixed<TType | undefined>(
+    typeof spec === 'function' ? { check: spec } : spec,
+  );
 }
 // XXX: this is using the Base schema so that `addMethod(mixed)` works as a base class
 create.prototype = Mixed.prototype;
diff --git a/src/number.ts b/src/number.ts
index fda598674..7ef4597df 100644
--- a/src/number.ts
+++ b/src/number.ts
@@ -32,7 +32,14 @@ export default class NumberSchema<
   TFlags extends Flags = '',
 > extends BaseSchema<TType, TContext, TDefault, TFlags> {
   constructor() {
-    super({ type: 'number' });
+    super({
+      type: 'number',
+      check(value: any): value is NonNullable<TType> {
+        if (value instanceof Number) value = value.valueOf();
+
+        return typeof value === 'number' && !isNaN(value);
+      },
+    });
 
     this.withMutation(() => {
       this.transform(function (value) {
@@ -52,12 +59,6 @@ export default class NumberSchema<
     });
   }
 
-  protected _typeCheck(value: any): value is NonNullable<TType> {
-    if (value instanceof Number) value = value.valueOf();
-
-    return typeof value === 'number' && !isNaN(value);
-  }
-
   min(min: number | Reference<number>, message = locale.min) {
     return this.test({
       message,
diff --git a/src/object.ts b/src/object.ts
index 207cf1796..7065f82f9 100644
--- a/src/object.ts
+++ b/src/object.ts
@@ -118,6 +118,9 @@ export default class ObjectSchema<
   constructor(spec?: Shape<TIn, TContext>) {
     super({
       type: 'object',
+      check(value): value is NonNullable<MakeKeysOptional<TIn>> {
+        return isObject(value) || typeof value === 'function';
+      },
     });
 
     this.withMutation(() => {
@@ -139,12 +142,6 @@ export default class ObjectSchema<
     });
   }
 
-  protected _typeCheck(
-    value: any,
-  ): value is NonNullable<MakeKeysOptional<TIn>> {
-    return isObject(value) || typeof value === 'function';
-  }
-
   protected _cast(_value: any, options: InternalOptions<TContext> = {}) {
     let value = super._cast(_value, options);
 
diff --git a/src/schema.ts b/src/schema.ts
index e9cb37aee..ae65debcf 100644
--- a/src/schema.ts
+++ b/src/schema.ts
@@ -53,9 +53,10 @@ export type SchemaSpec<TDefault> = {
   meta?: any;
 };
 
-export type SchemaOptions<TDefault> = {
+export type SchemaOptions<TType, TDefault> = {
   type?: string;
   spec?: SchemaSpec<TDefault>;
+  check?: (value: any) => value is NonNullable<TType>;
 };
 
 export type AnySchema<
@@ -148,10 +149,11 @@ export default abstract class BaseSchema<
   protected _blacklist = new ReferenceSet();
 
   protected exclusiveTests: Record<string, boolean> = Object.create(null);
+  protected _typeCheck: (value: any) => value is NonNullable<TType>;
 
   spec: SchemaSpec<any>;
 
-  constructor(options?: SchemaOptions<any>) {
+  constructor(options?: SchemaOptions<TType, any>) {
     this.tests = [];
     this.transforms = [];
 
@@ -160,6 +162,8 @@ export default abstract class BaseSchema<
     });
 
     this.type = options?.type || ('mixed' as const);
+    this._typeCheck =
+      options?.check || ((v: any): v is NonNullable<TType> => true);
 
     this.spec = {
       strip: false,
@@ -181,10 +185,6 @@ export default abstract class BaseSchema<
     return this.type;
   }
 
-  protected _typeCheck(_value: any): _value is NonNullable<TType> {
-    return true;
-  }
-
   clone(spec?: Partial<SchemaSpec<any>>): this {
     if (this._mutate) {
       if (spec) Object.assign(this.spec, spec);
@@ -197,6 +197,7 @@ export default abstract class BaseSchema<
 
     // @ts-expect-error this is readonly
     next.type = this.type;
+    next._typeCheck = this._typeCheck;
 
     next._whitelist = this._whitelist.clone();
     next._blacklist = this._blacklist.clone();
diff --git a/src/string.ts b/src/string.ts
index 7b74df035..e8766fc37 100644
--- a/src/string.ts
+++ b/src/string.ts
@@ -55,7 +55,14 @@ export default class StringSchema<
   TFlags extends Flags = '',
 > extends BaseSchema<TType, TContext, TDefault, TFlags> {
   constructor() {
-    super({ type: 'string' });
+    super({
+      type: 'string',
+      check(value): value is NonNullable<TType> {
+        if (value instanceof String) value = value.valueOf();
+
+        return typeof value === 'string';
+      },
+    });
 
     this.withMutation(() => {
       this.transform(function (value) {
@@ -72,12 +79,6 @@ export default class StringSchema<
     });
   }
 
-  protected _typeCheck(value: any): value is NonNullable<TType> {
-    if (value instanceof String) value = value.valueOf();
-
-    return typeof value === 'string';
-  }
-
   protected _isPresent(value: any) {
     return super._isPresent(value) && !!value.length;
   }
diff --git a/test/mixed.ts b/test/mixed.ts
index 784f396ff..5428da545 100644
--- a/test/mixed.ts
+++ b/test/mixed.ts
@@ -58,6 +58,35 @@ describe('Mixed Types ', () => {
     expect(inst.getDefault({ context: { foo: 'greet' } })).toBe('hi');
   });
 
+  it('should use provided check', async () => {
+    let schema = mixed((v): v is string => typeof v === 'string');
+
+    // @ts-expect-error narrowed type
+    schema.default(1);
+
+    expect(schema.isType(1)).toBe(false);
+    expect(schema.isType('foo')).toBe(true);
+
+    await expect(schema.validate(1)).rejects.toThrowError(
+      /this must match the configured type\. The validated value was: `1`/,
+    );
+
+    schema = mixed({
+      type: 'string',
+      check: (v): v is string => typeof v === 'string',
+    });
+
+    // @ts-expect-error narrowed type
+    schema.default(1);
+
+    expect(schema.isType(1)).toBe(false);
+    expect(schema.isType('foo')).toBe(true);
+
+    await expect(schema.validate(1)).rejects.toThrowError(
+      /this must be a `string` type/,
+    );
+  });
+
   it('should warn about null types', async () => {
     await expect(string().strict().validate(null)).rejects.toThrowError(
       /this cannot be null/,
diff --git a/test/types/types.ts b/test/types/types.ts
index 5d7e7070d..63ea70bad 100644
--- a/test/types/types.ts
+++ b/test/types/types.ts
@@ -90,6 +90,15 @@ Mixed: {
 
   // $ExpectType "foo" | undefined
   mixed<string>().notRequired().concat(string<'foo'>()).cast('');
+
+  // $ExpectType MixedSchema<string | undefined, AnyObject, undefined, "">
+  mixed((value): value is string => typeof value === 'string');
+
+  // $ExpectType MixedSchema<string | undefined, AnyObject, undefined, "">
+  mixed({
+    type: 'string',
+    check: (value): value is string => typeof value === 'string',
+  });
 }
 
 Strings: {