在数学里，单射函数（或称嵌射函数、一对一函数，英文称 injection、injective function或 one-to-one function）为一函数，其将不同的输入值对应到不同的函数值上。更精确地说，函数f被称为是单射的，当对每一陪域内的y，存在至多一个定义域内的x使得f(x) = y。

另一种说法为，f为单射，当若f(a) = f(b)，则a = b（或若a ≠ b，则f(a) ≠ f(b)），其中a, b属于定义域。

In mathematics, an injective function or injection or one-to-one function is a function that preserves distinctness: it never maps distinct elements of its domain to the same element of its codomain. In other words, every element of the function's codomain is the image of at most one element of its domain. The term one-to-one function must not be confused with one-to-one correspondence (aka bijective function), which uniquely maps all elements in both domain and codomain to each other, (see figures).