ARTICLE ABSTRACTHistone deacetylase (HDAC) inhibition has sporadic clinical efficacy in urothelial carcinoma; the genomic basis for clinical response is not known. In two separate phase I clinical trials testing pharmacokinetic aspects of HDAC inhibitors in advanced solid tumors, we identified one patient with advanced urothelial carcinoma who had a complete response to belinostat, and one patient with advanced urothelial carcinoma who had a partial response to panobinostat. The archived tumors of the responders were genomically characterized in comparison to others with urothelial carcinoma on the trials. Urothelial carcinoma cell lines treated with panobinostat and belinostat were studied to elucidate the mechanisms of benefit. Notably, the urothelial carcinoma tumors that responded to HDAC inhibition had ARID1A mutations. ARID1A mutations were also noted in the tumors of three patients who had stable disease as their best response to HDAC inhibition. Corroborating the basis of sensitivity, transcriptional profiling of platinum-resistant ARID1A-mutated HT1197 cells treated with panobinostat reveals negative enrichment for both cyto-proliferative (MYC and E2F targets) and DNA repair gene sets, and positive enrichment for TP53 and inflammatory gene sets. Our study identifies ARID1A loss as a basis for clinical response to pan HDAC inhibition and offers avenues for potential rational therapeutic combinations with HDAC inhibitors in advanced urothelial carcinoma.