Two-dimensional (2D) WO3 nanosheets exhibit a range of novel properties and functionalities that render them attractive for advanced nanotechnologies. However, at the ultimate 2D limit of single-layer thickness, the structural properties of WO3 are unclear. Here, we fabricated, using molecular beam epitaxy techniques, a crystalline 2D WO3 overlayer on a Ag(100) surface and unveiled its geometric, electronic, and vibrational structure via a combination of state-of-the-art experimental (microscopic and spectroscopic) and computational techniques. The 2D WO3 phase forms a bilayer with a staggered arrangement of WO6 octahedra, linked together by corner- and edge-sharing, which is significantly different from the cubic and monoclinic WO3 bulk structures, but resembles a bilayer of the α-MoO3 layered bulk lattice. Such a 2D WO3 bilayer on Ag(100) is a robust nonpolar structure, which is incommensurate in various rotational orientations, weakly coupled to the metal substrate, and, according to the density functional theory calculations, should survive as a stable freestanding layer, that is, as a nanosheet.