Hematite (α-Fe2O3)-based photoanodes offer great potential for use in solar hydrogen production as part of efforts to construct a sustainable and renewable energy economy based on photoelectrochemical (PEC) water splitting. A co-doping modification is of the utmost significance for improving PEC performance. To develop an efficient photoanode, a comprehensive grasp of co-dopants with diverse valence states is necessary. Herein, we describe a hydrothermal and dip-coating approach to the fabrication of Hf-doped Fe2O3 (Hf-HT) photoanodes co-doped with Be2+, Al3+, Si4+, and Nb5+ and evaluate the influence of each co-dopant on PEC performance. The PEC characteristic results revealed that Be2+ and Al3+ co-dopants enhanced surface charge separation efficiency, thus accelerating charge transfers at the photoanode–electrolyte interface. Meanwhile, the PEC performance of the Hf-HT photoanode co-doped with Nb did not significantly improve because of the thick Nb2O5 overlayer. However, the use of a Si4+ co-dopant improved the bulk properties of the photoanode. An optimized Hf-HT photoanode co-doped with Be achieved a photocurrent density of 1.98 mA/cm2 at 1.23 VRHE. This demonstrates that ex situ co-doping can have both positive and negative impacts on the PEC activity of photoelectrodes, and the co-dopants used to accomplish the desired outcomes should be considered in detail.