Mytilidae are a family of small to large marine and brackish-water bivalve molluscs in the order Mytilida. One of the genera, Limnoperna, even inhabits freshwater environments. The order has only this one family which contains some 52 genera.
Species in the family Mytilidae 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.
A 2020 study of the phylogeny of Mytilidae recovered two main clades derived from an epifaunal ancestor, with subsequent lineages shifting to other lifestyles, and correlating convergent evolution of siphon traits.