Cyclins and cyclin-dependent kinases (CDKs) are hyperactivated in nearly all human tumor types. To identify new approaches for interfering with cyclins/CDKs, we systematically searched for microRNAs (miRNAs) regulating these proteins. We uncovered a group of miRNAs that target nearly all cyclins and CDKs, and demonstrated that these miRNAs are very effective in shutting off cancer cell expansion. By profiling the response of over 120 human cancer cell lines representing 12 tumor types to these cell-cycle-targeting miRNAs, we identified miRNAs particularly effective against triple-negative breast cancers and KRAS-mutated cancers. We also derived expression-based algorithm that predicts response of primary tumors to cell-cycle-targeting miRNAs. Using systemic administration of nanoparticle-formulated miRNAs, we halted tumor progression in seven mouse xenograft models, including three highly aggressive and treatment-refractory patient-derived tumors, without affecting normal tissues. Our results highlight the utility of using cell-cycle-targeting miRNAs for treatment of refractory cancer types.