We present magnetization isotherms in pulsed magnetic fields up to 62 T, supported by first-principles calculations, demonstrating a huge uniaxial magnetocrystalline anisotropy energy\char22{}approximately 20 $\text{MJ}/{\mathrm{m}}^{3}$\char22{}in ${\mathrm{UMn}}_{2}{\mathrm{Ge}}_{2}$. This large anisotropy results from the extremely strong spin-orbit coupling affecting the uranium $5f$ electrons, which in the calculations exhibit a substantial orbital moment exceeding $2{$μ${}}_{B}$. We also find from theoretical calculations that a number of isostructural Mn actinide compounds are expected to have similarly large anisotropy.