Background In the preplanned interim analysis of the TOPAZ-1 study, durvalumab plus gemcitabine-cisplatin significantly improved overall survival versus placebo plus gemcitabine-cisplatin in participants with advanced biliary tract cancer. We aimed to report updated overall survival and safety data from TOPAZ-1 with additional follow-up and data maturity beyond the interim analysis. Methods TOPAZ-1 was a phase 3, randomised, double-masked, placebo-controlled, global study done at 105 sites in 17 countries. Participants aged 18 years or older with unresectable, locally advanced, or metastatic biliary tract cancer were randomly assigned (1:1) to durvalumab plus gemcitabine-cisplatin or placebo plus gemcitabine-cisplatin using a computer-generated randomisation scheme, stratified by disease status and primary tumour location. Participants received durvalumab (1500 mg) or placebo on day 1 of each cycle every 3 weeks for up to eight cycles, plus gemcitabine (1000 mg/m 2 ) and cisplatin (25 mg/m 2 ) intravenously on days 1 and 8 of each cycle every 3 weeks for up to eight cycles, followed by durvalumab (1500 mg) or placebo monotherapy every 4 weeks until disease progression or other discontinuation criteria were met. Investigators and participants were masked to study treatment. The primary endpoint was overall survival. TOPAZ-1 met its primary endpoint at the preplanned interim analysis, and the study is active but no longer recruiting participants. Updated overall survival and safety data from TOPAZ-1, with additional follow-up (data cutoff Feb 25, 2022) and data maturity beyond the interim analysis, are reported here. Efficacy was assessed in the full analysis set (all randomly assigned participants). Safety was assessed in the safety analysis set (all participants who received at least one dose of study treatment). The TOPAZ-1 study is registered with ClinicalTrials.gov, NCT03875235. Findings From April 16, 2019, to Dec 11, 2020, 914 participants were enrolled, 685 of whom were randomly assigned (341 to the durvalumab plus gemcitabine-cisplatin group and 344 to the placebo plus gemcitabine-cisplatin group). 345 (50%) participants were male and 340 (50%) were female. Median follow-up at the updated data cutoff was 23<middle dot>4 months (95% CI 20<middle dot>6-25<middle dot>2) in the durvalumab plus gemcitabine-cisplatin group and 22<middle dot>4 months (21<middle dot>4-23<middle dot>8) in the placebo plus gemcitabine-cisplatin group. At the updated data cutoff, 248 (73%) participants in the durvalumab plus gemcitabine-cisplatin group and 279 (81%) participants in the placebo plus gemcitabine-cisplatin group had died (median overall survival 12<middle dot>9 months [95% CI 11<middle dot>6-14<middle dot>1] vs 11<middle dot>3 months [10<middle dot>1-12<middle dot>5]; hazard ratio 0<middle dot>76 [95% CI 0<middle dot>64-0<middle dot>91]). Kaplan-Meier-estimated 24-month overall survival rates were 23<middle dot>6% (95% CI 18<middle dot>7-28<middle dot>9) in the durvalumab plus gemcitabine-cisplatin group and 11<middle dot>5% (7<middle dot>6-16<middle dot>2) in the placebo plus gemcitabine-cisplatin group. Maximum grade 3 or 4 adverse events occurred in 250 (74%) of 338 participants in the durvalumab plus gemcitabine-cisplatin group and 257 (75%) of 342 in the placebo plus gemcitabine-cisplatin group. The most common maximum grade 3 or 4 treatment-related adverse events were decreased neutrophil count (70 [21%] vs 86 [25%]), anaemia (64 [19%] vs 64 [19%]), and neutropenia (63 [19%] vs 68 [20%]).