Importance: Use of postoperative antimicrobial prophylaxis is common in pediatric surgery despite consensus guidelines recommending discontinuation following incision closure. The association between postoperative prophylaxis use and surgical site infection (SSI) in children undergoing surgical procedures remains poorly characterized. Objective: To evaluate whether use of postoperative surgical prophylaxis is correlated with SSI rates in children undergoing nonemergent surgery. Design, Setting, and Participants: This is a multicenter cohort study using 30-day postoperative SSI data from the American College of Surgeons' Pediatric National Surgical Quality Improvement Program (ACS NSQIP-Pediatric) augmented with antibiotic-use data obtained through supplemental medical record review from June 2019 to June 2021. This study took place at 93 hospitals participating in the ACS NSQIP-Pediatric Surgical Antibiotic Prophylaxis Stewardship Collaborative. Participants were children (<18 years of age) undergoing nonemergent surgical procedures. Exclusion criteria included antibiotic allergies, conditions associated with impaired immune function, and preexisting infections requiring intravenous antibiotics at time of surgery. Exposures: Continuation of antimicrobial prophylaxis beyond time of incision closure. Main Outcomes and Measures: Thirty-day postoperative rate of incisional or organ space SSI. Hierarchical regression was used to estimate hospital-level odds ratios (ORs) for SSI rates and postoperative prophylaxis use. SSI measures were adjusted for differences in procedure mix, patient characteristics, and comorbidity profiles, while use measures were adjusted for clinically related procedure groups. Pearson correlations were used to examine the associations between hospital-level postoperative prophylaxis use and SSI measures. Results: Forty thousand six hundred eleven patients (47.3% female; median age, 7 years) were included, of which 41.6% received postoperative prophylaxis (hospital range, 0%-71.2%). Odds ratios (ORs) for postoperative prophylaxis use ranged 190-fold across hospitals (OR, 0.10-19.30) and ORs for SSI rates ranged 4-fold (OR, 0.55-1.90). No correlation was found between use of postoperative prophylaxis and SSI rates overall (r = 0.13; P =.20), and when stratified by SSI type (incisional SSI, r = 0.08; P =.43 and organ space SSI, r = 0.13; P =.23), and surgical specialty (general surgery, r = 0.02; P =.83; urology, r = 0.05; P =.64; plastic surgery, r = 0.11; P =.35; otolaryngology, r = -0.13; P =.25; orthopedic surgery, r = 0.05; P =.61; and neurosurgery, r = 0.02; P =.85). Conclusions and Relevance: Use of postoperative surgical antimicrobial prophylaxis was not correlated with SSI rates at the hospital level after adjusting for differences in procedure mix and patient characteristics..