In mathematics, a reflection formula or reflection relation for a function f is a relationship between f(a − x) and f(x). It is a special case of a functional equation. It is common in mathematical literature to use the term "functional equation" for what are specifically reflection formulae.

Reflection formulae are useful for numerical computation of special functions. In effect, an approximation that has greater accuracy or only converges on one side of a reflection point (typically in the positive half of the complex plane) can be employed for all arguments.

The even and odd functions satisfy by definition simple reflection relations around a = 0. For all even functions,

$${\displaystyle f(-x)=f(x),}$$

and for all odd functions,

$${\displaystyle f(-x)=-f(x).}$$

A famous relationship is Euler's reflection formula

$${\displaystyle \Gamma (z)\Gamma (1-z)={\frac {\pi }{\sin {(\pi z)}}},\qquad z\not \in \mathbb {Z} }$$