DE CVn is a cool M-dwarf (about 0.51 solar masses, 3,579 K), 30.5 light-years from Earth. DE CVn b is a 12.0-Jupiter-mass companion, with an orbital period of 11.2 years, at 5.750 AU from its star, discovered in 2018 via eclipse timing variations. DE CVn adds another data point to the growing catalogue of planetary systems whose masses, orbits, and host-star properties constrain models of planet formation. DE CVn b circles in the outer system, comparable to the Jupiter region. At 12.0 Jupiter masses, the companion lies near the dividing line between giant planets and brown dwarfs.