BACKGROUND: Standard treatment for high-risk non-muscle-invasive bladder cancer is transurethral resection of bladder tumour followed by intravesical BCG immunotherapy. However, despite high initial responses rates, up to 50% of patients have recurrence or become BCG-unresponsive. PD-1 pathway activation is implicated in BCG resistance. In the KEYNOTE-057 study, we evaluated pembrolizumab, a PD-1 inhibitor, in BCG-unresponsive non-muscle-invasive bladder cancer. METHODS: We did this open-label, single-arm, multicentre, phase 2 study in 54 sites (hospitals and cancer centres) in 14 countries. In cohort A of the trial, adults aged 18 years or older with histologically confirmed BCG-unresponsive carcinoma in situ of the bladder, with or without papillary tumours, with an Eastern Cooperative Oncology Group performance status of 0-2, and who were ineligible for or declined radical cystectomy were enrolled. All enrolled patients were assigned to receive pembrolizumab 200 mg intravenously every 3 weeks for up to 24 months or until centrally confirmed disease persistence, recurrence, or progression; unacceptable toxic effects; or withdrawal of consent. The primary endpoint was clinical complete response rate (absence of high-risk non-muscle-invasive bladder cancer or progressive disease), assessed by cystoscopy and urine cytology approximately 3 months after the first dose of study drug. Patient follow-ups were done every 3 months for the first 2 years and every 6 months thereafter for up to 5 years. Efficacy was assessed in all patients who received at least one dose of the study drug and met BCG-unresponsive criteria. Safety was assessed in all patients who received at least one dose of the study drug. This trial is registered with ClinicalTrials.gov number, NCT02625961, and is ongoing. FINDINGS: Between Dec 9, 2015, and April 1, 2018, we screened 334 patients for inclusion. 186 patients did not meet inclusion criteria, and 47 patients were assigned to cohort B (patients with BCG-unresponsive high grade Ta or any grade T1 papillary disease without carcinoma in situ; results will be reported separately). 101 eligible patients were enrolled and assigned to receive pembrolizumab. All 101 patients received at least one dose of the study drug and were included in the safety analysis. Five patients had disease that did not meet the US Food and Drug Administration definition of BCG-unresponsive non-muscle-invasive bladder cancer and were therefore not included in the efficacy analysis (n=96). Median follow-up was 36·4 months (IQR 32·0-40·7). 39 (41%; 95% CI 30·7-51·1) of 96 patients with BCG-unresponsive carcinoma in situ of the bladder with or without papillary tumours had a complete response at 3 months. Grade 3 or 4 treatment-related adverse events occurred in 13 (13%) patients; the most common were arthralgia (in two [2%] patients) and hyponatraemia (in three [3%] patients). Serious treatment-related adverse events occurred in eight (8%) patients. There were no deaths that were considered treatment related. INTERPRETATION: Pembrolizumab monotherapy was tolerable and showed promising antitumour activity in patients with BCG-unresponsive non-muscle-invasive bladder cancer who declined or were ineligible for radical cystectomy and should be considered a a clinically active non-surgical treatment option in this difficult-to-treat population. FUNDING: Merck Sharp & Dohme.