Black crystals of the mixed-valence osmate(V,VI) Sr₄Os₃O₁₂ were grown via a gas phase reaction in an evacuated silica tube. The material is attracted to a permanent magnet at room temperature (295±5 K) but loses this property when heated or cooled. In this temperature interval, soft magnetic behavior with vanishing coercivity is observed, but the saturation magnetization is only 0.05 μ_B per formula unit. Resistivity measurements show that Sr₄Os₃O₁₂ is a semiconductor with a small band gap of about 0.26 eV at room temperature. X-ray diffraction on a single-crystal revealed a rhombohedral structure with the space group R (Formula presented.) m and lattice parameters a_r=554.64(2) pm and c_r=2700.1(1) pm at 300(1) K. The compound crystallizes in the La₄Ti₃O₁₂ structure type and is isostructural to Sr₄CoRe₂O₁₂. The structure is a rhombohedral stack of layered blocks, each of which can be considered as a section of three layers of the cubic perovskite structure cut perpendicular to one of its threefold axes. The Os–O−Os bond angle of about 175° favors superexchange that leads to antiferromagnetic coupling between the osmium atoms with 5d³ and 5d² configurations. Sr₄Os₃O₁₂ exhibits strong lattice dynamics. One aspect is the antiferro-rotative lattice modes of the corner-sharing [OsO₆] octahedra, which freeze upon cooling. At 100 K, an ordered, yet twinned triclinic superstructure is formed. Strong spin-orbit coupling and other effects cause deformations of the [OsO₆] octahedra that differ significantly at 300 K and 100 K, suggesting a change in the electronic configuration. Competing ground states could also explain the temperature-dependent band gap and the magnetic fluctuations manifested in the magnetization peak at room temperature.