Ceftolozane/tazobactam, a cephalosporin-β-lactamase inhibitor combination, active against multidrug-resistant Gram-negative pathogens, is approved for treatment of adults with complicated urinary tract infections (cUTI). Safety and efficacy of ceftolozane/tazobactam in pediatric participants with cUTI, including pyelonephritis, were assessed. This phase 2 study (NCT03230838) compared ceftolozane/tazobactam with meropenem for treatment of cUTI in participants from birth to <18 years of age. The primary objective was safety and tolerability. Key secondary end points included clinical cure and per-participant microbiologic response rates at end of treatment (EOT) and test of cure (TOC) visits. The microbiologic modified intent-to-treat (mMITT) population included 95 participants (ceftolozane/tazobactam, n = 71; meropenem, n = 24). The most common diagnosis and pathogen were pyelonephritis (ceftolozane/tazobactam, 84.5%; meropenem, 79.2%) and Escherichia coli (ceftolozane/tazobactam, 74.6%; meropenem, 87.5%); 5.7% (ceftolozane/tazobactam) and 4.8% (meropenem) of E. coli isolates were extended-spectrum β-lactamase-producers. Rates of adverse events were similar between treatment groups (any: ceftolozane/tazobactam, 59.0% vs. meropenem, 60.6%; drug-related: ceftolozane/tazobactam, 14.0% vs. meropenem, 15.2%; serious: ceftolozane/tazobactam, 3.0% vs. meropenem, 6.1%). Rates of clinical cure for ceftolozane/tazobactam and meropenem at EOT were 94.4% and 100% and at TOC were 88.7% and 95.8%, respectively. Rates of microbiologic eradication for ceftolozane/tazobactam and meropenem at EOT were 93.0% and 95.8%, and at TOC were 84.5% and 87.5%, respectively. Ceftolozane/tazobactam had a favorable safety profile in pediatric participants with cUTI; rates of clinical cure and microbiologic eradication were high and similar to meropenem. Ceftolozane/tazobactam is a safe and effective new treatment option for children with cUTI, especially due to antibacterial-resistant Gram-negative pathogens.