Abstract: Tungsten ditelluride is a layered transition-metal dichalcogenide that crystalizes in a distorted hexagonal net with an orthorhombic unit cell (Td phase). The lack of inversion symmetry in this phase leads to a predicted new topological semimetal. Here, we use a single THz pulse to trigger a structural phase transition from the orthorhombic structure to a different metastable phase of this material in a reversible manner, and probe the switching using an ultrafast electron diffraction technique. Related studies in the alloy MoWTe2 as well as ultrafast measurements of the atomic-scale response in the just below switching threshold regime will also be presented. These findings serve as the first direct evidence of a THz field induced structural transition in a two-dimensional material, and offer a new promising way to optically control the topological properties of solids.