We report the passivation of the Mn acceptors in Ga _1-x Mn _x P upon exposure to a hydrogen plasma. The as-grown films are nonmetallic and ferromagnetic with a Curie temperature of T_C=55 K . After hydrogenation the sample resistivity increases by approximately three orders of magnitude at room temperature and six orders of magnitude at 25 K. Furthermore, the hydrogenated samples are paramagnetic, which is evidenced by a magnetization curve at 5 K that is best described by a Brillouin function with g=2 and J=5/2 expected for Mn atoms in the 3d⁵ configuration. Upon annealing, partial depassivation and a recovery of ferromagnetism are observed. These observations unambiguously demonstrate that the ferromagnetism in Ga _1-x Mn _x P is carrier-mediated similar to Ga _1-x Mn _x As .