Nanostructured H₂ evolution cocatalysts are able to promote charge separation and thus enhance the efficiency of the photocatalytic H₂ evolution reaction (HER). However, the nanosized cocatalyst particles are easily detached from the surfaces of semiconductors or severely aggregated in reaction systems, which not only greatly reduces the photocatalytic HER efficiency during long-term use but also greatly increases the difficulty of recovery. Moreover, powdery cocatalysts have poor compatibility with the scale-up photoelectrochemical devices. In this paper, a monolithic cocatalyst is developed by controllably growing Ni₂P nanowire arrays on Ni foam substrate (Ni₂P NWAs/NF) via a direct vapor-phase phosphorization method. The grown Ni₂P NWAs with high specific surface areas can not only offer ample active sites for the HER, but also serve as scaffolds for anchoring dye molecules to maximize the light utilization efficiency, which endows the Ni₂P NWAs/NF monolithic cocatalyst with excellent HER activity. When sensitized with Erythrosin B (ErB) in triethanolamine (TEOA) solution, the turnover number (TON) of H₂ evolution based on ErB reaches 9.7 in 5 h under visible light. Notably, the good structural integrity and inherent magnetism enable the Ni₂P NWAs/NF to be easily separated from the reaction solution and excellent catalytic H₂ evolution stability over a 45 h cycling reaction. This work presents a new strategy of fabricating monolithic cocatalysts with controllable microstructure and functionalities as well as high activity, durability, and device-compatibility for large-scale solar energy conversion applications.