Searching photocatalysts for efficient hydrogen production has been a challenging issue for solar-energy harvesting. Using co-catalyst is proven to be an effective approach to improve the efficiency of photocatalyst in water-splitting. Here, we report that carbonized MoS₂ (MoS₂/Mo₂C) can be a superactive co-catalyst in solar-driven hydrogen production. We show that MoS₂/Mo₂C decorated CdS achieves a high photocatalytic hydrogen evolution rate (34 mmol/h/g, ∼112 times higher than pure CdS) and excellent apparent quantum efficiency (41.4% at 420 nm). The outstanding photocatalytic performance of MoS₂/Mo₂C/CdS is attributed to the metallic characteristic of MoS₂/Mo₂C and suitable Gibbs free energy of hydrogen adsorption, leading to enhanced light absorption, fast separation and transportation of photoinduced carriers, and optimal activity in hydrogen evolution reaction (HER). We further show that MoS₂/Mo₂C as co-catalyst can also dramatically improve the photocatalytic activity of g-C₃N₄. Our findings demonstrate that the carbonized transition metal disulfide can be active as co-catalyst in photocatalysis, providing guidance on exploring novel photocatalysts for energy harvesting.