Two-dimensional (2D) transition metal dichalcogenides are regarded as promising candidates for nanoelectronic devices, due to their novel electronic properties. Motivated by the similarity of valence electrons between Mo and B2 pairs, we design a new type of 2D MoS2-like material, i.e. boron dichalcogenides, through the global minimization search and density functional theory methods. Free standing trigonal and hexagonal phase boron dichalcogenide (B2X2, X = S, Se and Te) monolayers are predicted to be highly stable. The T-B2X2 and H-B2X2 monolayers are all semiconducting with indirect bandgaps ranging from 2.14 eV to 4.01 eV and large excitonic effects. Particularly, H-B2X2 exhibits high carrier mobility of up to 6.23 × 105 cm2 (V−1 s−1), which can be comparable to that of graphene. Therefore, 2D boron dichalcogenides have great potential for applications in high-performance flexible field-effect transistors and light emitters. Furthermore, the optical spectrum of these monolayers reveals that the absorption is in the ultraviolet region, suggesting future applications in ultraviolet optoelectronic devices.