The PsbP is a thylakoid lumenal subunit of photosystem II (PSII), which has developed specifically in higher plants and green algae. In higher plants, the molecular function of PsbP has been intensively investigated by release-reconstitution experiments in vitro. Recently, solution of a high-resolution structure of PsbP has enabled investigation of structure-function relationships, and efficient gene-silencing techniques have demonstrated the crucial role of PsbP in PSII activity in vivo. Furthermore, genomic and proteomic studies have shown that PsbP belongs to the divergent PsbP protein family, which consists of about 10 members in model plants such as Arabidopsis and rice. Characterization of the molecular function of PsbP homologs using Arabidopsis mutants suggests that each plays a distinct and important function in maintaining photosynthetic electron transfer. In this review, recent findings regarding the molecular functions of PsbP and other PsbP homologs in higher plants are summarized, and the molecular evolution of these proteins is discussed.