Two-dimensional chalcogenide monolayers are strong candidates for next-generation flexible and transparent optoelectronics. Due to the intrinsic ultrathin thickness and limited optical absorption, however, their responsivity is normally low. Here we develop a simple and low-cost method to fabricate high-performance hybrid phototransistors of monolayer WS2 with significantly enhanced responsivity and an extended spectral response range, by virtue of surface decoration with liquid-phase exfoliated SnS nanosheets (NSs). The hybrid phototransistors show a much enhanced responsivity of ∼2 A W-1 and an ultrahigh light/dark signal-to-noise ratio of 106 under 457 nm excitation, exhibiting a significant increase of 3 orders of magnitude in responsivity and a 100 fold increase in signal-to-noise ratio, compared with pure WS2 devices. Our hybrid photodetectors also exhibit a respectable response speed, with a rise and decay time of 51 μs and 98 μs, respectively. After optimal surface decoration with narrow bandgap SnS NSs atop a monolayer WS2 channel, an emergent optical responsivity in the near infrared region (1064 nm) is also observed.