Treatment of jaundiced newborns with subthreshold phototherapy (phototherapy given to newborns with bilirubin levels below those recommended in American Academy of Pediatrics [AAP] guidelines) is common. However, the use of subthreshold phototherapy may have risks and increase costs, and, to date, it has not been systematically studied in newborns. To estimate the efficacy of subthreshold phototherapy for newborns with total serum bilirubin (TSB) levels from 0.1 to 3.0 mg/dL below the appropriate AAP phototherapy threshold during the birth hospitalization in preventing readmissions for phototherapy, and to identify predictors of readmission for phototherapy. Retrospective cohort study of 25 895 newborns born at 35 or more weeks’ gestation, born in 1 of 16 Kaiser Permanente Northern California hospitals from January 1, 2010, through December 31, 2014, with at least 1 TSB level from 0.1 to 3.0 mg/dL below the appropriate AAP phototherapy threshold and not exceeding the threshold during the birth hospitalization. Data were analyzed from November 1, 2015, to November 28, 2017. Subthreshold phototherapy during the birth hospitalization. Readmission for phototherapy. Among 25 895 newborns with qualifying TSB levels from 0.1 to 3.0 mg/dL below the appropriate AAP phototherapy threshold, 4956 (19.1%) received subthreshold phototherapy and 241 of these (4.9%) were readmitted for phototherapy compared with 2690 of 20 939 untreated newborns (12.8%) (unadjusted odds ratio [OR], 0.35; 95% CI, 0.30-0.40). In a logistic regression model, adjustment for confounding variables, including gestational age, race/ethnicity, formula feedings per day, and the difference between the TSB level and the phototherapy threshold, strengthened the association (OR, 0.28; 95% CI, 0.19-0.40). Estimated numbers needed to treat ranged from 60.8 in the lowest quintile of predicted risk to 6.3 in the highest quintile. Newborns who received formula feedings had lower adjusted odds of readmission for phototherapy compared with exclusively breastfed newborns (OR, 0.58; 95% CI, 0.47-0.72 for >0 to <2 formula feedings per day; OR, 0.24; 95% CI, 0.21-0.27 for ≥6 formula feedings per day). Subthreshold phototherapy was associated with a 22-hour longer length of stay (95% CI, 16-28 hours). Subthreshold phototherapy during the birth hospitalization is effective in preventing readmissions for phototherapy; however, for each readmission prevented, many newborns require phototherapy who would otherwise not need it.