In this study, we rediscovered the tRNA synthetase AARS1 to be a moonlighting but bona fide lactyltransferase that can directly use lactate as a donor of lactyl-group and ATP as an energy source — and on that basis, we revealed a noncanonical function of lactate in tumor cells, i.e., to transmit a YAP-TEAD1–activating cell proliferation–promoting signal via lactylation, explaining from a new angle how tumors benefit from the Warburg effect. Here, YAP1 is linked to neoplasm.