Abstract Purpose: TAK-659 is an investigational, dual SYK/FLT3 inhibitor with preclinical activity in B-cell malignancy models. This first-in-human, dose-escalation/expansion study aimed to determine the safety, tolerability, MTD/recommended phase II dose (RP2D), and preliminary efficacy of TAK-659 in relapsed/refractory solid tumors and B-cell lymphomas. Patients and Methods: Patients received continuous, once-daily oral TAK-659, 60–120 mg in 28-day cycles, until disease progression or unacceptable toxicity. The study applied an accelerated dose-escalation design to determine the MTD and RP2D. In the expansion phase, patients with lymphoma were enrolled in five disease cohorts at the MTD. Results: Overall, 105 patients were enrolled [dose escalation, n = 36 (solid tumors, n = 19; lymphoma, n = 17); expansion, n = 69]. The MTD was 100 mg once daily. TAK-659 absorption was fast (Tmax ∼2 hours) with a long terminal half-life (∼37 hours). Exposure generally increased with dose (60–120 mg), with moderate variability. The most common treatment-related adverse events were generally asymptomatic and reversible elevations in clinical laboratory values. Among 43 response-evaluable patients with diffuse large B-cell lymphoma, 8 (19%) achieved a complete response (CR) with an overall response rate (ORR) of 28% [23% intent-to-treat (ITT)]. Responses were seen in both de novo and transformed disease and appeared independent of cell-of-origin classification. Among 9 response-evaluable patients with follicular lymphoma, 2 (22%) achieved CR with an ORR of 89% (57% ITT). Conclusions: TAK-659 has single-agent activity in patients with B-cell lymphoma. Further studies of the drug in combination, including an evaluation of the biologically optimal and safest long-term dose and schedule, are warranted.