BRCA1 is an important gene involved in susceptibility to breast and ovarian cancer and its product regulates the cellular response to DNA double‐strand breaks. Here, we present evidence that BRCA1 also contributes to the transcription‐coupled repair (TCR) of ultraviolet (UV) light‐induced DNA damage. BRCA1 immediately accumulates at the sites of UV irradiation‐mediated damage in cell nuclei in a manner that is fully dependent on both Cockayne syndrome B (CSB) protein and active transcription. Suppression of BRCA1 expression inhibits the TCR of UV lesions and increases the UV sensitivity of cells proficient in TCR. BRCA1 physically interacts with CSB protein. BRCA1 polyubiquitinates CSB and this polyubiquitination and subsequent degradation of CSB occur following UV irradiation, even in the absence of Cockayne syndrome A (CSA) protein. The depletion of BRCA1 expression increases the UV sensitivity of CSA‐deficient cells. These results indicate that BRCA1 is involved in TCR and that a BRCA1‐dependent polyubiquitination pathway for CSB exists alongside the CSA‐dependent pathway to yield more efficient excision repair of lesions on the transcribed DNA strand. (Cancer Sci 2011; 102: 1840–1847)