Layered transition metal dichalcogenides (TMDs) have attracted renewed interest owing to their potential use as two-dimensional components in next-generation devices. Although group 6 TMDs, such as MX 2 with M = (Mo, W) and X = (S, Se, Te), can exist in several polymorphs, most studies have been conducted with the semiconducting hexagonal (2H) phase as other polymorphs often exhibit inhomogeneous formation. Here, we report a reversible structural phase transition between the hexagonal and stable monoclinic (distorted octahedral or 1T′) phases in bulk single-crystalline MoTe 2. Furthermore, an electronic phase transition from semimetallic to semiconducting is shown as 1T′-MoTe 2 crystals go from bulk to few-layered. Bulk 1T′-MoTe 2 crystals exhibit a maximum carrier mobility of 4,000 cm 2 V -1 s -1 and a giant magnetoresistance of 16,000% in a magnetic field of 14 T at 1.8 K. In the few-layered form, 1T′-MoTe 2 exhibits a bandgap opening of up to 60 meV, which our density functional theory calculations identify as arising from strong interband spin-orbit coupling. We further clarify that the Peierls distortion is a key mechanism to stabilize the monoclinic structure. This class of semiconducting MoTe 2 unlocks the possibility of topological quantum devices based on non-trivial Z 2 -band-topology quantum spin Hall insulators in monoclinic TMDs (ref.).