We measured the high-field magnetization up to 55 T and constructed a magnetic phase diagram for a transuranium, antiferromagnet NpRhGa5 with the tetragonal structure. The magnetization at 4.2K for H parallel to[100] indicates a sharp metamagnetic transition with a step at H-c = 26T and saturates above H-s = 38T, reaching 0.43 mu(B)/Np. An ordered moment of 0.96 mu(B)/Np at zero field, which was determined from the neutron scattering experiment, is found to be reduced to 0.43 mu(B)/Np at H-s, together with an orientation of the magnetic moment from the (001) plane to the (100) plane. (c) 2006 Elsevier B.V. All rights reserved.