We present a genome assembly from an adult colony of Membranipora membranacea (the sea mat; Bryozoa; Gymnolaemata; Cheilostomatida; Membraniporidae). The genome sequence is 339 megabases in span. Most of the assembly (99.95\%) is scaffolded into 11 chromosomal pseudomolecules. The mitochondrial genome has also been assembled and is 14.7 kilobases in length.