Two-dimensional transition metal dichalcogenide (TMD) based heterostructures are attractive candidates for optoelectronic nanodevice applications. Here, both photoelectrocatalytic (PEC) water splitting and surface-enhanced Raman scattering (SERS) performances are largely improved for a lateral/vertical 1T-2H MoS₂/Au heterostructure. Combined experimental and performance studies unambiguously reveal that the efficient PEC performance of the 1T-2H MoS₂/Au heterostructure mainly originates from the formation of a lateral/vertical heterojunction for efficient light utilization and fast charge transfer. Meanwhile, such a heterostructure exhibits excellent SERS detection sensitivity towards rhodamine 6G (R6G), with a detection limit as low as 10⁻¹⁰ M and an enhancement factor (EF) of 8.1 × 10⁶, owing to the coupling effect of the electromagnetic effect from plasmonic Au nanoparticles and the efficient charge transfer process between the heterostructure and the detected molecules. This study not only reveals the origin of the PEC and SERS performance enhancements based on the lateral/vertical 1T-2H MoS₂/Au heterostructure, but also paves the way for designing multifunctional optoelectronic nanodevices.