The ferromagnetic semiconductor $\mathrm{B}{\mathrm{a}}_{2}\mathrm{NiOs}{\mathrm{O}}_{6}$ (${T}_{\mathrm{mag}}\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) was synthesized at 6 GPa and 1500 \ifmmode^\circ\else\textdegree\fi{}C. It crystallizes into a double perovskite structure [$Fm\ensuremath{-}3m$; $a=8.0428(1)\phantom{\rule{0.16em}{0ex}}\AA{}$], where the $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{O}{\mathrm{s}}^{6+}$ ions are perfectly ordered at the perovskite $B$ site. We show that the spin-orbit coupling of $\mathrm{O}{\mathrm{s}}^{6+}$ 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 $>21\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$ 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 (${T}_{\mathrm{mag}}<180\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), the spin-gapless semiconductor $\mathrm{M}{\mathrm{n}}_{2}\mathrm{CoAl}$ (${T}_{\mathrm{mag}}\ensuremath{\sim}720\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), and the ferromagnetic insulators EuO (${T}_{\mathrm{mag}}\ensuremath{\sim}70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) and $\mathrm{B}{\mathrm{i}}_{3}\mathrm{C}{\mathrm{r}}_{3}{\mathrm{O}}_{11}$ (${T}_{\mathrm{mag}}\ensuremath{\sim}220\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of $\mathrm{B}{\mathrm{a}}_{2}\mathrm{NiOs}{\mathrm{O}}_{6}$ should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.