  We show that the thermal relic abundance of dark matter can be affected by a
new type of reaction: semi-annihilation. Semi-annihilation takes the schematic
form X_i X_j -> X_k phi, where X_i are stable dark matter particles and phi is
an unstable state. Such reactions are generically present when dark matter is
composed of more than one species with "flavor" and/or "baryon" symmetries. We
give a complete set of coupled Boltzmann equations in the presence of
semi-annihilations, and study two toy models featuring this process.
Semi-annihilation leads to non-trivial dark matter dynamics in the early
universe, often dominating over ordinary annihilation in determining the relic
abundance. This process also has important implications for indirect detection
experiments, by enriching the final state spectrum from dark matter
(semi-)annihilation in the Milky Way.
