The recent discovery of large numbers of phytochrome photoreceptor genes in both photosynthetic and non-photosynthetic prokaryotes has led to efforts to understand their physiological roles in environmental acclimation. One receptor in this class, RcaE, is involved in controlling complementary chromatic adaptation, a process that regulates the transcription of operons encoding light-harvesting proteins in cyanobacteria. Although all previously identified phytochrome responses are maximally sensitive to red and far red light, complementary chromatic adaptation is unique in that it is responsive to green and red light. Here, we present biochemical and genetic evidence demonstrating that RcaE is a photoreceptor and that it requires the cysteine at position 198 to ligate an open chain tetrapyrrole covalently in a manner analogous to chromophore attachment in plant phytochromes. Furthermore, although the wild-type rcaE gene can rescue red and green light photoresponses of an rcaE null mutant, a gene in which the codon for cysteine 198 is converted to an alanine codon rescues the red light but not the green light response. Thus, RcaE is a photoreceptor that is required for both green and red light responsiveness during complementary chromatic adaptation and is the first identified phytochrome class sensor that is involved in sensing and responding to green and red light rather than red and far red light.