The two-component signal transduction pathway widespread in prokaryotes, fungi, molds, and some plants involves an elaborate phosphorelay cascade. Rcp1 is the phosphate receiver module in a two-component system controlling the light response of cyanobacteria Synechocystis sp. via cyanobacterial phytochrome Cph1, which recognizes Rcp1 and transfers its phosphoryl group to an aspartate residue in response to light. Here we describe the crystal structure of Rcp1 refined to a crystallographic R-factor of 18.8% at a resolution of 1.9 A. The structure reveals a tightly associated homodimer with monomers comprised of doubly wound five-stranded parallel beta-sheets forming a single-domain protein homologous with the N-terminal activator domain of other response regulators (e.g., chemotaxis protein CheY). The three-dimensional structure of Rcp1 appears consistent with the conserved activation mechanism of phosphate receiver proteins, although in this case, the C-terminal half of its regulatory domain, which undergoes structural changes upon phosphorylation, contributes to the dimerization interface. The involvement of the residues undergoing phosphorylation-induced conformational changes at the dimeric interface suggests that dimerization of Rcp1 may be regulated by phosphorylation, which could affect the interaction of Rcp1 with downstream target molecules.