Adriamycin (ADM)‐based regimens are the most effective chemotherapeutic treatments for breast cancer. However, intrinsic and acquired chemoresistance is a major therapeutic problem. Our goal was to clarify the role of mediator complex subunit 19 (Med19) in chemotherapy resistance and to elucidate the related molecular mechanisms. In this study, ADM‐resistant human cells (MCF‐7/ADM) and tissues exhibited increased Med19 expression and autophagy levels relative to the corresponding control groups. Additionally, MCF‐7/ADM cells showed changes in two selective markers of autophagy. There was a dose‐dependent increase in the light chain 3 (LC3)‐II/LC3‐I ratio and a decrease in sequestosome 1 (P62/SQSTMl) expression. Furthermore, lentivirus‐mediated Med19 inhibition significantly attenuated the LC3‐II/LC3‐I ratio, autophagy‐related gene 3 (Atg3) and autophagy‐related gene 5 (Atg5) expression, P62 degradation, and red fluorescent protein‐LC3 dot formation after treatment with ADM or rapamycin, an autophagy activator. Furthermore, the antiproliferative effects of ADM, cisplatin (DDP), and taxol (TAX) were significantly enhanced after suppressing Med19 expression. Notably, the effects of Med19 on autophagy were mediated through the high‐mobility group box‐1 (HMGB1) pathway. Our findings suggest that Med19 suppression increased ADM chemosensitivity by downregulating autophagy through the inhibition of HMGB1 signaling in human breast cancer cells. Thus, the regulatory mechanisms of Med19 in autophagy should be investigated to reduce tumor resistance to chemotherapy.