Members of the genus Marinomonas in the Gammaproteobacteria are broadly distributed in marine environments where they could be infected by bacteriophages. Here we report the genome sequence of bacteriophage P12026 that can lytically infect bacterial strain IMCC12026, a member of the genus Marinomonas. To our knowledge, this is the first genome sequence of a lytic bacteriophage infecting the genus Marinomonas.