Background: The diffuse large B-cell lymphoma (DLBCL) variant of Richter transformation (DLBCL-RT) is typically chemoresistant with poor prognosis. Aiming to explore a chemotherapy-free treatment combination that triggers anti-tumour immune responses, we conducted a phase 2 study of atezolizumab (a PD-L1 inhibitor) in combination with venetoclax and obinutuzumab in patients with DLBCL-RT., Methods: This was a prospective, open-label, multicentre, single-arm, investigator-initiated, phase 2 study in 15 hospitals in Italy and Switzerland. Eligible patients had a confirmed diagnosis of chronic lymphocytic leukaemia or small lymphocytic lymphoma as per the International Workshop on Chronic Lymphocytic Leukemia (IWCLL) 2008 criteria with biopsy-proven transformation to DLBCL; had not previously received treatment for DLBCL-RT, although they could have received chronic lymphocytic leukaemia therapies; were aged 18 years or older; and had an Eastern Cooperative Oncology Group (ECOG) performance status of 0-2. No previous treatment with any of the drugs in the triplet combination was allowed. Patients received 35 cycles of 21 days of intravenous obinutuzumab (100 mg on day 1, 900 mg on day 2, 1000 mg on day 8 and day 15 of cycle 1; 1000 mg on day 1 of cycles 2-8) and intravenous atezolizumab (1200 mg on day 2 of cycle 1 and 1200 mg on day 1 of cycles 2-18), and continuous oral venetoclax (ramp-up from 20 mg/day on day 15 of cycle 1 according to chronic lymphocytic leukaemia schedule, then 400 mg/day from day 1 of cycle 3 to day 21 of cycle 35). The primary endpoint was overall response rate at day 21 of cycle 6 in the intention-to-treat population. We considered an overall response rate of 67% or more to be clinically active, rejecting the null hypothesis of a response of 40% or less. The study is registered with ClinicalTrials.gov, NCT04082897, and has been completed., Findings: Between Oct 9, 2019, and Oct 19, 2022, 28 patients were enrolled (12 [43%] male patients and 16 [57%] female patients). Median follow-up was 16·8 months (IQR 7·8-32·0). At cycle 6, 19 of 28 patients showed a response, yielding an overall response rate of 67·9% (95% CI 47·6-84·1). Treatment-emergent adverse events that were grade 3 or worse were reported in 17 (61%; 95% CI 40·6-78·5) of 28 patients, with neutropenia being the most frequent (11 [39%; 21·5-59·4] of 28 patients). Serious treatment-emergent adverse events were reported in eight (29%; 14·2-48·7) patients, which were most commonly infections (five [18%; 6·1-36·9] of 28 patients). There were two (7%) deaths attributable to adverse events during the study: one from sepsis and one from fungal pneumonia, which were not considered as directly treatment-related by the investigators. Six (21·4%) patients had immune-related adverse events, none of which led to discontinuation. No tumour lysis syndrome was observed., Interpretation: The atezolizumab, venetoclax, and obinutuzumab triplet combination was shown to be active and safe, suggesting that this chemotherapy-free regimen could become a new first-line treatment approach in patients with DLBCL-RT., Funding: Roche., Competing Interests: Declaration of interests AT reports payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Janssen, Beigene, and Abbvie; support for attending meetings or travel from Abbvie, Janssen, and Beigene; and participation on a data safety monitoring board or advisory board from Janssen, Abbvie, Beigene, Lilly, and AstraZeneca. AMF reports consulting fees from Janssen; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Janssen, Beigene, and AstraZeneca; support for attending meetings or travel from Abbvie, Janssen, Beigene, and AstraZeneca; and participation on a data safety monitoring board or advisory board from Janssen, Beigene, and AstraZeneca. ACo reports grants or contracts from Gilead, BMS, Beigene, Abbvie, Janssen-Cilag, and AstraZeneca; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from AstraZeneca, Abbvie, and Janssen-Cilag; support for attending meetings or travel from Janssen; and participation on a data safety monitoring board or advisory board from BMS. MC reports grants or contracts from Johnson & Johnson and Abbvie; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Abbvie and AstraZeneca; support for attending meetings or travel from Johnson & Johnson, Abbvie, and AstraZeneca; and participation on a data safety monitoring board or advisory board from Johnson & Johnson, Abbvie, AstraZeneca, Beigene, and GSK. RC reports support for the present manuscript from the US National Institutes of Health and National Cancer Institute (R01CA196703–01). PLZ reports consulting fees and payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Takeda, BMS, MSD, Roche, Gilead, Novartis, Abbvie, Beigene, Kyowa Kirin, and Janssen. MMot reports payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events; support for attending meetings or travel; and participation on a data safety monitoring board or advisory board from Abbvie, Janssen, and AstraZeneca. GG reports grants from Associazione Italiana per la Ricerca sul Cancro and Piano Nazionale di Ripresa e Resilienza; consulting fees from Abbvie, AstraZeneca, Beiene, Hikma, and Johnson & Johnson; and participation on a data safety monitoring board or advisory board from Abbvie, AstraZeneca, Beigene, Hikma, Johnson & Johnson, and Lilly. LS reports consulting from Abbvie, Beigene, Lilly, AstraZeneca, Janssen, and Merck; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Abbvie, Beigene, Lilly, Octapharma, AstraZeneca, Janssen, and Merck; support for attending meetings or travel from AstraZeneca, Janssen, and Beigene; and participation on a data safety monitoring board or advisory board from Merck. RCa reports payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Abbvie, Gilead, Gentilipharma, Pierre Fabre, Celgene, and Menarini stemline; support for attending meetings or travel from Pierre Fabre, Beigene, and Servier; and participation on a data safety monitoring board or advisory board from Celgene, Abbvie, Gentilipharma, and Daichi Samkio. TZ report consulting fees and payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Roche, Novartis, Gilead, Janssen, AstraZeneca, Lilly, and Abbvie. EZ reports consulting fees from Abbvie, Beigene, BMS, Cures, Elly and Lilly, Incyte, Ipsen, Merck, Miltenyibiomedicine, and Roche; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Abbvie, AstraZeneca, Beigene, and Gilead; support for attending meetings or travel from Abbvie; and participation on a data safety monitoring board or advisory board from Merck. DR reports support for the present manuscript from Adaptive; grants or contracts, consulting fees, payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Abbvie, AstraZeneca, Beigene, BMS, Janssen, and Lilly. All other authors declare no competing interests. No honoraria or payments were made for authorship., (Copyright © Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)