在计算机科学中，子类型（subtype）一般是同另一个数据类型（超类型，supertype）有关系的数据类型，通过了某种可代换性概念，这意味着为在这个超类型的元素上运算而写计算机进程也可以在子类型的元素上运算。更明确的说，超类型-子类型关系经常通过Liskov代换原则来定义；但是任何给定编程语言都可以定义它自己的子类型概念，或根本没有这个概念。

子类型这一概念也称作“子类型多态”（subtype polymorphism）或者“包含多态”（inclusion polymorphism）。在面向对象进程设计语言中，多态一般是指子类型多态，而参数多态一般称作泛型进程设计。

In programming language theory, subtyping (also subtype polymorphism or inclusion polymorphism) is a form of type polymorphism in which a subtype is a datatype that is related to another datatype (the supertype) by some notion of substitutability, meaning that program elements, typically subroutines or functions, written to operate on elements of the supertype can also operate on elements of the subtype. If S is a subtype of T, the subtyping relation is often written S <: T, to mean that any term of type S can be safely used in a context where a term of type T is expected. The precise semantics of subtyping crucially depends on the particulars of what "safely used in a context where" means in a given programming language. The type system of a programming language essentially defines its own subtyping relation, which may well be trivial.