The magnetism of Pd1 − xMnx is investigated theoretically. A localized spin model for Mn spins that interact withshort-range antiferromagnetic interactions and long-range ferromagnetic interactions viaitinerant d electrons is set up, with no adjustable parameters. A multicanonicalMonte Carlo simulation, combined with a procedure of symmetry breaking, isemployed to discriminate between the ferromagnetic and spin glass orders. Thetransition temperature and the low-temperature phase are determined from thetemperature variation of the specific heat and the probability distributions of theferromagnetic order parameter and the spin glass order parameter at differentconcentrations. The calculation results reveal that only the ferromagnetic phase exists atx < 0.02, that only the spinglass phase exists at x > 0.04, and that the two phases coexist at intermediate concentrations. This result agreessemi-quantitatively with experimental results.