Recent studies show that annotated long noncoding RNAs (lncRNAs) and circular RNAs (circRNAs) encode for stable, functional peptides that contribute to human development and disease. To systematically discover lncRNAs and circRNAs encoding peptides, we performed a comprehensive integrative analysis of mass spectrometry-based proteomic and transcriptomic sequencing data from >900 patients across nine cancer types. This enabled us to identify 19,871 novel peptides derived from 8,903 lncRNAs. Further, we exploited open reading frames overlapping the backspliced region of circRNAs to identify 3,238 peptides that are uniquely derived from 2,834 circRNAs and not their corresponding linear RNAs. Collectively, our pan-cancer proteogenomic analysis will serve as a resource for evaluating the coding potential of lncRNAs and circRNAs that could aid future mechanistic studies exploring their function in cancer.