BACKGROUND: Urinary tract infections (UTIs) are among the most prevalent microbial diseases and their financial burden on society is substantial. In the context of increasing antibiotic resistance, finding alternative treatments for UTIs is a top priority. We aimed to determine whether intravesical bacteriophage therapy with a commercial bacteriophage cocktail is effective in treating UTI. METHODS: We did a randomised, placebo-controlled, clinical trial, at the Alexander Tsulukidze National Centre of Urology, Tbilisi, Georgia. Men older than 18 years of age, who were scheduled for transurethral resection of the prostate (TURP), with complicated UTI or recurrent uncomplicated UTI but no signs of systemic infection, were allocated by block randomisation in a 1:1:1 ratio to receive intravesical Pyo bacteriophage (Pyophage; 20 mL) or intravesical placebo solution (20 mL) in a double-blind manner twice daily for 7 days, or systemically applied antibiotics (according to sensitivities) as an open-label standard-of-care comparator. Urine culture was taken via urinary catheter at the end of treatment (ie, day 7) or at withdrawal from the trial. The primary outcome was microbiological treatment response after 7 days of treatment, measured by urine culture; secondary outcomes included clinical and safety parameters during the treatment period. Analyses were done in a modified intention-to-treat population of patients having received at least one dose of the allocated treatment regimen. This trial is registered with ClinicalTrials.gov, NCT03140085. FINDINGS: Between June 2, 2017, and Dec 14, 2018, 474 patients were screened for eligibility and 113 (24%) patients were randomly assigned to treatment (37 to Pyophage, 38 to placebo, and 38 to antibiotic treatment). 97 patients (28 Pyophage, 32 placebo, 37 antibiotics) received at least one dose of their allocated treatment and were included in the primary analysis. Treatment success rates did not differ between groups. Normalisation of urine culture was achieved in five (18%) of 28 patients in the Pyophage group compared with nine (28%) of 32 patients in the placebo group (odds ratio [OR] 1·60 [95% CI 0·45-5·71]; p=0·47) and 13 (35%) of 37 patients in the antibiotic group (2·66 [0·79-8·82]; p=0·11). Adverse events occurred in six (21%) of 28 patients in the Pyophage group compared with 13 (41%) of 32 patients in the placebo group (OR 0·36 [95% CI 0·11-1·17]; p=0·089) and 11 (30%) of 37 patients in the antibiotic group (0·66 [0·21-2·07]; p=0·47). INTERPRETATION: Intravesical bacteriophage therapy was non-inferior to standard-of-care antibiotic treatment, but was not superior to placebo bladder irrigation, in terms of efficacy or safety in treating UTIs in patients undergoing TURP. Moreover, the bacteriophage safety profile seems to be favourable. Although bacteriophages are not yet a recognised or approved treatment option for UTIs, this trial provides new insight to optimise the design of further large-scale clinical studies to define the role of bacteriophages in UTI treatment. FUNDING: Swiss Continence Foundation, the Swiss National Science Foundation, and the Swiss Agency for Development and Cooperation. TRANSLATIONS: For the Georgian and German translations of the abstract see Supplementary Materials section.