Layered perovskite-type semiconductor La2Ti2O7 has attracted lots of attention in photocatalytic hydrogen evolution, due to the suitable energy band position for water splitting, high specific surface area, and excellent physicochemical stability. However, the narrow light absorption range and the low separation efficiency of photogenerated carriers limit its photocatalytic activity. Herein, plate-like La2Ti2O7 with uniform crystal morphology was synthesized in molten NaCl salt. A p-n heterojunction was then constructed through the in situ hydrothermal growth of p-type Co3O4 nanoparticles on the surface of n-type plate-like La2Ti2O7. The effects of Co3O4 loading on photocatalytic hydrogen evolution performance were investigated in detail. The results demonstrate that composite Co3O4/La2Ti2O7 possesses much better photocatalytic activity than the pure component. The composite photocatalyst with 1 wt% Co3O4 exhibits the highest hydrogen evolution rate of 79.73 μmol·g−1·h−1 and a good cycling stability. The photoelectrochemistry characterizations illustrate that the improvement of photocatalytic activity is mainly attributed to both the enhanced light absorption from the Co3O4 ornament and the rapid separation of photogenerated electron-hole pairs driven by the built-in electric field close to the p-n heterojunction. The results may provide further insights into the design of high-efficiency La2Ti2O7-based heterojunctions for photocatalytic hydrogen evolution.