Reduced cancer incidence has been reported among type II diabetics treated with metformin. Metformin exhibits anti-proliferative and anti-neoplastic effects associated with inhibition of mTORC1, but the mechanisms are poorly understood. We provide the first genome-wide analysis of translational targets of canonical mTOR inhibitors (rapamycin and PP242) and metformin, revealing that metformin controls gene expression at the level of mRNA translation to an extent comparable to that of canonical mTOR inhibitors. Importantly, metformin's anti-proliferative activity can be explained by selective translational suppression of mRNAs encoding cell cycle regulators via the mTORC1/4E-BP pathway. Thus, metformin selectively inhibits mRNA translation of encoded proteins that promote neoplastic proliferation, motivating further studies of this compound and related biguanides in cancer prevention and treatment.