In mathematics, the **additive identity** of a set that is equipped with the operation of addition is an element which, when added to any element x in the set, yields x. One of the most familiar additive identities is the number 0 from elementary mathematics, but additive identities occur in other mathematical structures where addition is defined, such as in groups and rings.

- The additive identity familiar from elementary mathematics is zero, denoted 0. For example,
- In the natural numbers (if 0 is included), the integers the rational numbers the real numbers and the complex numbers the additive identity is 0. This says that for a number n belonging to any of these sets,

Let N be a group that is closed under the operation of addition, denoted +. An additive identity for N, denoted e, is an element in N such that for any element n in N,

- In a group, the additive identity is the identity element of the group, is often denoted 0, and is unique (see below for proof).
- A ring or field is a group under the operation of addition and thus these also have a unique additive identity 0. This is defined to be different from the multiplicative identity 1 if the ring (or field) has more than one element. If the additive identity and the multiplicative identity are the same, then the ring is trivial (proved below).
- In the ring M
_{m × n}(*R*) of m-by-n matrices over a ring R, the additive identity is the zero matrix,^{[1]}denoted**O**or**0**, and is the m-by-n matrix whose entries consist entirely of the identity element 0 in R. For example, in the 2×2 matrices over the integers the additive identity is - In the quaternions, 0 is the additive identity.
- In the ring of functions from , the function mapping every number to 0 is the additive identity.
- In the additive group of vectors in the origin or zero vector is the additive identity.

Let (*G*, +) be a group and let 0 and 0' in G both denote additive identities, so for any g in G,

It then follows from the above that

In a system with a multiplication operation that distributes over addition, the additive identity is a multiplicative absorbing element, meaning that for any s in S, *s* · 0 = 0. This follows because:

Let R be a ring and suppose that the additive identity 0 and the multiplicative identity 1 are equal, i.e. 0 = 1. Let r be any element of R. Then

proving that R is trivial, i.e. *R* = {0}. The contrapositive, that if R is non-trivial then 0 is not equal to 1, is therefore shown.