MicroRNAs (miRNAs) contributes to almost all physiological and pathological processes. It is well established that miRNAs control gene expression through a post-transcriptional mechanism that involves mRNA degradation and translation repression.20, 21, 22, 23 miRNAs have crucial function in cancer; more than 50% of miRNA genes are located at chromosomal regions that include fragile sites or regions that exhibit genetic deletions or amplifications.24, 25, 26 miR-17/92 is the best-characterized oncogenic miRNA cluster.27 Human miR-17/92 gene is frequently amplified in solid tumors and several hematopoietic malignancies.28, 29 We examined the expression patterns of miR-20a, a member of the miR-17/92 cluster, from TCGA breast cancer data set. A significant upregulation of miR-20a is observed in breast cancer, especially in triple-negative breast cancer. Gene Set Enrichment Analysis (GESA) suggests that miR-20a negatively correlates with the autophagy/lysosome pathway. Studies herein show that miR-20a inhibits basal and nutrient starvation-induced autophagic flux and lysosomal proteolytic activity, increases cellular reactive oxygen species (ROS) levels and DNA damage response by downregulating several key regulators of autophagy, including BECN1, ATG16L1 and SQSTM1. Low levels of BECN1, ATG16L1 and SQSTM1 are predominantly observed in triple-negative cancers than in other subtypes. A large fraction of copy-number-altered genome and DNA mutations are also detected in breast cancer patients with elevated levels of miR-20a. Collectively, our results suggest that miR-20a-mediated dysfunction of autophagy contributes to mammary tumorigenesis by promoting genomic damage and instability.