Mytilida is an order of marine bivalve molluscs, commonly known as true mussels. There is one extant superfamily, the Mytiloidea, with a single extant family, the Mytilidae.
Species in the order Mytilida are found worldwide, but they are more abundant in colder seas, where they often form uninterrupted beds on rocky shores in the intertidal zone and the shallow subtidal. The subfamily Bathymodiolinae is found in deep-sea habitats.
Mytilids include the well-known edible sea mussels.
A common feature of the shells of mussels is an asymmetrical shell which has a thick, adherent periostracum. The animals attach themselves to a solid substrate using a byssus.