Thirty-day readmission after congenital heart surgery (CHS) is an important outcome given the vulnerability of pediatric patients. We hypothesized that readmissions after pediatric CHS are common and identifiable risk factors exist. We obtained State Inpatient Databases for Washington, New York, Florida, and California and selected CHS admissions age < 19 years. The main outcome was readmission defined as non-elective hospitalization < 31 days of discharge from index CHS admission. In multivariable analyses using generalized estimating equations, we examined associations of patient-level characteristics (age, sex, race, household income, insurance status, genetic syndromes, co-morbidities, RACHS-1 surgical risk category and complication) and admission characteristics [weekend admission, urgent/emergent admission, and high resource use (HRU)] with 30-day pediatric readmission after adjusting for case mix. Among 8585 index admissions we identified 967 readmissions (11.3%). Median length of stay for readmissions was 5 days, median total charge of $31,973, and mortality rate 1.8%. Among readmissions, 1.7% underwent another CHS of which 44% were HRU, complication rate 88% and mortality 6.25%. In multivariable analysis, age 1 month–1 year AOR 1.3 p = 0.01; Hispanic ethnicity AOR 1.2 p = 0.03; government-insurance AOR 1.3 p = 0.01; RACHS-1 3 complexity AOR 2.4 p < 0.001; RACHS-1 4 + complexity 2.0 p = 0.001; HRU AOR 1.4 p = 0.02; complications AOR 1.1 p = 0.04; and emergent index admission AOR 2.0 p < 0.001 were risk factors for readmission. Over 11% of pediatric CHS admissions result in an unplanned readmission. Hispanic ethnicity, government insurance, HRU admissions, higher case complexity, complications, and emergent index admission are risk factors for readmission.