The transition metal dichalcogenides (TMDCs) have been intensively investigated as promising nanoelectronic and optoelectronic materials. However, the pervasive adsorbates on the surfaces of monolayer TMDCs, including oxygen and water molecules from the ambient environment, tend to degrade the device performance, thus hindering specific applications. In this work, we report the effect of laser irradiation on the transport and photoresponse of monolayer MoS2 and WSe2 devices, and this laser annealing process is demonstrated as a straightforward approach to remove physically adsorbed contaminants. Compared to vacuum pumping and in situ thermal annealing treatments, the field-effect transistors after laser annealing show a more than one order of magnitude higher on-state current, and no apparent degradation of device performance at low temperatures. The mobility of the monolayer WSe2 devices can be enhanced by three to four times, and for single-layered MoS2 devices with the commonly used SiO2 as the back-gate, the mobility increases by 20 times, reaching [Formula: see text]. The efficient cleaning effect of laser annealing is also supported by the reduction of channel and contact resistance revealed by a transmission line experiment. Further, an enhanced photocurrent, by a factor of ten, has been obtained in the laser annealed device. These findings pave the way for high-performance monolayer TMDC-based electronic and optoelectronic devices with a clean surface and intrinsic properties.