Broadband photodetection is crucial for various defense and scientific applications such as biomedical imaging, communications, and environmental and spectral monitoring. In recent years, transition metal dichalcogenide semiconductors, from the two-dimensional layered materials family, have attracted special attention for their application in photodetection due to their outstanding optoelectronic properties and large optical absorbance for their atomically thin thicknesses. Here, we present a CVD-synthesized MoS2 phototransistor with Au/Ti contacts enhanced by Au nanoparticles via surface nanoplasmonics. From electronic and optoelectronic characterizations, intrinsic device parameters were extracted and analyzed including the field-effect mobility of 37.4 cm2V-1s-1, a high ON/OFF ratio of 106. Next, the optoelectronic characterization was carried out before and after Au nanoparticles using a tunable laser with a wavelength absorption range from 400 nm to 1100 nm under vacuum conditions. The spectral photoresponse was improved from a cutoff wavelength of ~ 975 nm before the Au nanoparticles to a broadband spectral detection with a minimum standard deviation of 0.56 μA at from near-ultraviolet to near-infrared and a maximum photocurrent of 7.61 μA at an incident optical power density of ~ 2 μWcm-2. In addition, the photocurrent has been increased 5-fold after decorating the MoS2 photodetector with Au nanoparticles. The improvement of the light-matter interaction of MoS2 nanosheet, described before, is attributed to the localized surface plasmon in gold nanoparticles.