Purpose: In this phase I study (NCT01307267), we evaluated safety, pharmacokinetics, clinical activity, and pharmacodynamics of treatment with utomilumab plus rituximab in patients with relapsed/refractory follicular lymphoma (FL) and other CD20+ non-Hodgkin lymphomas (NHL). Patients and Methods: Primary objectives were to assess treatment safety and tolerability for estimating the MTD, using a modified time-to-event continual reassessment method, and selecting the recommended phase II dose (RP2D). Results: Sixty-seven patients received utomilumab (0.03–10.0 mg/kg every 4 weeks) and rituximab (375 mg/m2 weekly) in the dose-escalation groups or utomilumab (1.2 mg/kg every 4 weeks) plus rituximab in the dose-expansion cohort. No patient experienced dose-limiting toxicity. The MTD for utomilumab in combination with rituximab was not reached and estimated to be ≥10 mg/kg every 4 weeks. The majority of the utomilumab treatment-related adverse events (AE) were grade 1 to 2; the most common AE was fatigue (16.4%). The pharmacokinetics of utomilumab in combination with rituximab was linear in the 0.03 to 10 mg/kg dose range. A low incidence (1.5%) of treatment-induced antidrug antibodies against utomilumab was observed. The objective response rate was 21.2% (95% CI, 12.1%–33.0%) in all patients with NHL, including four complete and 10 partial responses. Analysis of paired biopsies from a relapsed/refractory FL patient with complete response showed increased T-cell infiltration and cytotoxic activity in tumors. Biomarker correlations with outcomes suggested that clinical benefit may be contingent on patient immune function. Conclusions: Utomilumab in combination with rituximab demonstrated clinical activity and a favorable safety profile in patients with CD20+ NHLs.