Purpose: We conducted a first-in-human, dose-escalation study, to evaluate the safety, tolerability, pharmacokinetics, pharmacodynamics, and activity of TAK-931, a cell division cycle 7 inhibitor, in Japanese patients with advanced solid tumors. Experimental Design: Patients aged ≥20 years received oral TAK-931: once-daily (QD) for 14 days in 21-day cycles (schedule A; from 30 mg); QD or twice-daily for 7 days on, 7 days off in 28-day cycles (schedule B; from 60 mg); continuous QD (schedule D; from 20 mg); or QD for 2 days on, 5 days off (schedule E; from 100 mg) in 21-day cycles. Results: Of the 80 patients enrolled, all had prior systemic treatment and 86% had stage IV disease. In schedule A, two patients experienced dose-limiting toxicities (DLTs) of grade 4 neutropenia and the maximum tolerated dose (MTD) was 50 mg. In schedule B, four patients experienced DLTs of grade 3 febrile neutropenia (n = 3) or grade 4 neutropenia (n = 1); the MTD was 100 mg. Schedules D and E were discontinued before MTD determination. The most common adverse events were nausea (60%) and neutropenia (56%). Time to maximum plasma concentration of TAK-931 was ~1–4 hours post-dose; systemic exposure was approximately dose-proportional. Posttreatment pharmacodynamic effects correlating to drug exposure were observed. Overall, 5 patients achieved a partial response. Conclusions: TAK-931 was tolerable with a manageable safety profile. TAK-931 50 mg QD days 1–14 in 21-day cycles was selected as a recommended phase II dose and achieved proof of mechanism. Trial registration ID: NCT02699749