Cryptic transcription initiation has been previously linked to activation of oncogenic transcripts. However, the prevalence and impact of cryptic antisense transcription from the opposite strand of protein-coding genes were mostly unknown in cancer. Applying a robust computational pipeline to publicly available transcriptome and epigenome datasets, we identified hundreds of previously unannotated cryptic antisense polyadenylated transcripts (CAPTs) that were enriched in tumor samples. We showed that the activation of cryptic antisense transcription was associated with increased chromatin accessibility and active histone marks. Accordingly, we found that many of the antisense transcripts were inducible by treatment of epigenetic drugs. Moreover, CRISPR-mediated epigenetic editing assays revealed that transcription of a noncoding RNA LRRK1-CAPT promoted LUSC cell proliferation, suggesting its oncogenic role. Our findings largely expand our understanding of cancer-associated transcription events, which may facilitate the development of novel strategies for cancer diagnosis and treatment.