Catalytic MoS x thin films have been directly photoelectrodeposited on GaInP2 photocathodes for stable photoelectrochemical hydrogen generation. Specifically, the MoS x deposition conditions were controlled to obtain 8–10 nm films directly on p-GaInP2 substrates without ancillary protective layers. The films were nominally composed of MoS2, with additional MoO x S y and MoO3 species detected and showed no long-range crystalline order. The as-deposited material showed excellent catalytic activity toward the hydrogen evolution reaction relative to bare p-GaInP2. Notably, no appreciable photocurrent reduction was incurred by the addition of the photoelectrodeposited MoS x catalyst to the GaInP2 photocathode under light-limited operating conditions, highlighting the advantageous optical properties of the film. The MoS x catalyst also imparted enhanced durability toward photoelectrochemical hydrogen evolution in acidic conditions, maintaining nearly 85% of the initial photocurrent after 50 h of electrolysis. In total, this work demonstrates a simple method for producing dual-function catalyst/protective layers directly on high-performance, planar III–V photoelectrodes for photoelectrochemical energy conversion.