The development of efficient cocatalysts indicates one of the most significant topics for the commercialization of photoelectrochemical hydrogen evolution. The design and preparation of carbon‐based catalysts is a significant research part for inexpensive, environment‐friendly, and durable catalysts that may replace previously developed noble metal catalysts. Nevertheless, the challenge associated with the previous carbon‐based catalysts, such as graphene and graphene‐based materials, lies in the complicated and inefficient synthetic processes. Here, we suggest a biomimetic carbon nanosheet (CNS) as a new platform of hydrogen evolution catalyst for photoelectrochemical cell systems. The CNS is synthesized from polydopamine, which is an adhesive protein secreted by mussels. Active reaction sites for the hydrogen evolution in the CNS boost its catalytic activity with controllability. This significant enhancement of the photoelectrochemical performance with long‐term stability emphasizes the new potential for the application of biomimetic materials as a hydrogen evolution reaction catalyst as well as a molecular precursor.