Abstract Cisplatin-based chemotherapy is the standard of care for patients with muscle-invasive urothelial carcinoma. Pathologic downstaging to pT0/pTis after neoadjuvant cisplatin-based chemotherapy is associated with improved survival, although molecular determinants of cisplatin response are incompletely understood. We performed whole-exome sequencing on pretreatment tumor and germline DNA from 50 patients with muscle-invasive urothelial carcinoma who received neoadjuvant cisplatin-based chemotherapy followed by cystectomy (25 pT0/pTis “responders,” 25 pT2+ “nonresponders”) to identify somatic mutations that occurred preferentially in responders. ERCC2, a nucleotide excision repair gene, was the only significantly mutated gene enriched in the cisplatin responders compared with nonresponders (q < 0.01). Expression of representative ERCC2 mutants in an ERCC2-deficient cell line failed to rescue cisplatin and UV sensitivity compared with wild-type ERCC2. The lack of normal ERCC2 function may contribute to cisplatin sensitivity in urothelial cancer, and somatic ERCC2 mutation status may inform cisplatin-containing regimen usage in muscle-invasive urothelial carcinoma. Significance: Somatic ERCC2 mutations correlate with complete response to cisplatin-based chemosensitivity in muscle-invasive urothelial carcinoma, and clinically identified mutations lead to cisplatin sensitivity in vitro. Nucleotide excision repair pathway defects may drive exceptional response to conventional chemotherapy. Cancer Discov; 4(10); 1140–53. ©2014 AACR. See related commentary by Turchi et al., p. 1118 This article is highlighted in the In This Issue feature, p. 1103