The ferromagnetic semiconductor Ba2NiOsO6 (T-mag similar to 100 K) was synthesized at 6 GPa and 1500 degrees C. It crystallizes into a double perovskite structure [Fm-3m; a = 8.0428(1) angstrom], where the Ni2+ and Os6+ ions are perfectly ordered at the perovskite B site. We show that the spin-orbit coupling of Os6+ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a > 21 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga, Mn) As and (Cd, Mn) Te (T-mag