We present a genome assembly from a specimen of Ctena decussata (Mollusca; Bivalvia; Lucinida; Lucinidae). The genome sequence has a total length of 1,658.05 megabases. Most of the assembly (97.83%) is scaffolded into 18 chromosomal pseudomolecules. The mitochondrial genome has also been assembled and is 53.28 kilobases in length. The genome of Candidatus Thiodiazotropha sp. CDECU1, a bacterium associated with C. decussata was also assembled,