Dysregulation of cyclin-dependent kinases is a hallmark of myeloma, and specifically, cdk5 inhibition can enhance the activity of proteasome inhibitors in vitro. Dinaciclib is a novel potent small molecule inhibitor of cyclin-dependent kinases (CDK)1, CDK2, CDK5, and CDK9. Patients with relapsed multiple myeloma and ≤5 prior lines of therapy, with measurable disease, were enrolled. Dinaciclib was administered on day 1 of a 21-day cycle at doses of 30 to 50 mg/m2. Overall, 27 evaluable patients were accrued; the median number of prior therapies was 4. The dose level of 50 mg/m2 was determined to be the maximally tolerated dose. The overall confirmed partial response rate (PR) was 3 of 27 (11%), including 1 patient at the 30 mg/m2 dose (1 very good PR VGPR) and 2 patients at the 40 mg/m2 dose (1 VGPR and 1 PR). In addition, 2 patients at the 50 mg/mg2 dose achieved a minimal response (clinical benefit rate, 19%). Leukopenia, thrombocytopenia, gastrointestinal symptoms, alopecia, and fatigue were the most common adverse events. The current study demonstrates single agent activity of dinaciclib in relapsed myeloma, with 2 patients achieving a deep response (VGPR) and 10 patients obtaining some degree of M protein stabilization or decrease. This trial was registered at www.clinicaltrials.gov as #NCT01096342. •Dinaciclib is a novel cdk inhibitor that demonstrates single agent activity in myeloma.•Dinaciclib has a safety profile that is easily manageable.