Summary Background Ibrutinib is approved in the EU, USA, and other countries for patients with mantle cell lymphoma who received one previous therapy. In a previous phase 2 study with single-agent ibrutinib, the proportion of patients who achieved an objective response was 68%; 38 (34%) of 111 patients had transient lymphocytosis. We hypothesised that adding rituximab could target mantle cell lymphoma cells associated with redistribution lymphocytosis, leading to more potent antitumour activity. Methods Patients with a confirmed mantle cell lymphoma diagnosis (based on CD20-positive and cyclin D1-positive cells in tissue biopsy specimens), no upper limit on the number of previous treatments received, and an Eastern Cooperative Oncology Group performance status score of 2 or less were enrolled in this single-centre, open-label, phase 2 study. Patients received continuous oral ibrutinib (560 mg) daily until progessive disease or unacceptable toxic effects. Rituximab 375 mg/m2 was given intravenously once per week for 4 weeks during cycle 1, then on day 1 of cycles 3–8, and thereafter once every other cycle up to 2 years. The primary endpoint was the proportion of patients who achieved an objective response in the intention-to-treat population and safety assessed in the as-treated population. The study is registered with ClinicalTrials.gov , number NCT01880567 , and is still ongoing, but no longer accruing patients. Findings Between July 15, 2013, and June 30, 2014, 50 patients were enrolled. Median age was 67 years (range 45–86), and the median number of previous regimens was three (range 1–9). At a median follow-up of 16·5 months (IQR 12·09–19·28), 44 (88%, 95% CI 75·7–95·5) patients achieved an objective response, with 22 (44%, 30·0–58·7) patients achieving a complete response, and 22 (44%, 30·0–58·7) a partial response. The only grade 3 adverse event in >=10% of patients was atrial fibrillation, which was noted in six (12%) patients. Grade 4 diarrhoea and neutropenia occurred in one patient each. Adverse events led to discontinuation of therapy in five (10%) patients (atrial fibrillation in three 6% patients, liver infection in one 2%, and bleeding in one 2%). Two patients died while on-study from cardiac arrest and septic shock; the latter was deemed possibly related to treatment. Interpretation Ibrutinib combined with rituximab is active and well tolerated in patients with relapsed or refractory mantle cell lymphoma. Our results provide preliminary evidence for the activity of this combination in clinical practice. A phase 3 trial is warranted for more definitive data. Funding Pharmacyclics LLC, an AbbVie Company.