​​​Monolayer MoS2 is a direct-gap two-dimensional semiconductor that exhibits strong electronhole interactions, leading to the formation of stable excitons and trions. Here we report the existence of efficient excitonexciton annihilation, a four-body interaction, in this material. Excitonexciton annihilation was identified experimentally in ultrafast transient absorption measurements through the emergence of a decay channel varying quadratically with exciton density. The rate of excitonexciton annihilation was determined to be (4.3 +/- 1.1) x 10 (2) cm(2)/s at room temperature.