Using first-principles calculations we examine the crystal structures and phase transitions of nitride perovskite ${\mathrm{LaWN}}_{3}$. Lattice dynamics calculations indicate that the ground-state structure belongs to space group $R3c$. Two competitive phase transition pathways are identified which are characterized by symmetry-adapted distortion modes. The results suggest that $R3c\phantom{\rule{4pt}{0ex}}{\mathrm{LaWN}}_{3}$ should be an excellent ferroelectric semiconductor, as its large spontaneous polarization of around 61 $\ensuremath{\mu}\mathrm{C}/{\mathrm{cm}}^{2}$ is comparable to that of ${\mathrm{PbTiO}}_{3}$, and its band gap is about 1.72 eV. Ferroelectricity is found to result from the $B$-site instability driven by hybridization between $\mathrm{W}\ensuremath{-}5d$ and $\mathrm{N}\ensuremath{-}2p$ orbitals. These properties make ${\mathrm{LaWN}}_{3}$ an attractive candidate material for use in ferroelectric memory devices and photovoltaic cells.