Multi-drug resistance (MDR) develops because cancer cells evade toxicity of several structurally unrelated drugs. Besides other mechanisms, MDR is linked to the overexpression of ATP Binding Cassette (ABC), transporters, among which ABCB1 is the best characterized one. Since overactivation of PI3K/Akt/mTOR plays a pivotal role in the growth of human cancers, we hypothesized whether dual PI3K and mTOR inhibitor, BEZ235 (BEZ, dactolisib) reverses resistance to doxorubicin (DOX). Ovarian (A2780) and pancreatic (MiaPaca2) cancer cells were used to generate DOX-resistant clones by overexpressing ABCB1 or stepwise treatment of DOX. Intracellular accumulation of DOX was measured by flow cytometry after treatment with BEZ. BEZ treatment caused an increase in intracellular levels of DOX which was almost identical to the naïve parental cell lines. BEZ was found to be a weak substrate for ABCB1 as demonstrated by minimal increase in ATPase activity. BEZ treatment caused a dose-dependent decrease in cell viability in combination with DOX, which was associated with an increase in cleaved PARP expression in the drug resistant clones. These results suggest that BEZ is a non-substrate inhibitor of ABCB1 and is able to effectively re-sensitize cells overexpressing ABCB1 to the effects of DOX. Dual PI3 Kinase/mTOR inhibitor, BEZ, has the potential to reverse MDR in cancer patients. •Multidrug Resistance (MDR) decreases effectiveness of cancer drugs.•Targeting ATP Binding Cassette (ABC) transporters is known to reverse MDR.•Dual PI3Kinase/mTOR Inhibitor BEZ235 increased doxorubicin accumulation in drug-resistant cancer cells.•BEZ235 improved doxorubicin-induced cancer cell killing.•MDR in cancer cells can be reversed by BEZ235.