Summary Plants monitor the light environment through informational photoreceptors that include phytochromes. In seedling de‐etiolation, phytochrome B (phyB), which is the most important member of the phytochrome family, interacts with transcription factors to regulate gene expression and transduce light signals. In this study, we identified rrc1 ( reduced red‐light responses in cry1cry2 background 1 ), an Arabidopsis mutant that is impaired in phyB‐mediated light responses. A genetic analysis demonstrated that RRC1 affected light signaling in a phyB‐dependent manner. RRC1 encodes an ortholog of the human potential splicing factor SR140. The RRC1 polypeptide contains a C‐terminal arginine/serine‐rich (RS) domain that is important for the regulation of alternative splicing. Although the complete loss of RRC1 caused pleiotropic developmental abnormalities, the deletion of the RS domain specifically reduced phyB signaling and caused aberrant alternative splicing of several SR protein genes. Moreover, semi‐quantitative RT‐PCR analysis revealed that the alternative splicing patterns of some of the SR protein genes were altered in a red‐light‐dependent manner, and that these responses were reduced in both phyB and rrc1 mutants. These findings suggest that the regulation of alternative splicing by the RS domain of RRC1 plays an important role in phyB signal transduction.