Preterm birth affects 12% of infants in the United States annually and is the main contributor to infant deaths and long-term neurological disabilities in offspring. Obesity is a growing problem in the U.S., and is increasingly being considered a major risk factor for adverse health outcomes. Puerto Rican teenagers have disproportionately high rates of preterm birth and obesity when compared to non-Hispanic White teenagers. Studies evaluating risk factors for preterm birth among adolescents are sparse, have inconsistent findings, and were conducted among predominantly non-Hispanic populations. Therefore, we investigated the association between BMI and preterm birth among the 419 teenage (ages 16-19) participants in Proyecto Buena Salud, a prospective cohort study of predominantly Puerto Rican prenatal care patients in Massachusetts. Pre-pregnancy BMI was abstracted from medical records and defined using CDC adolescent BMI-for-age percentile categories. Preterm birth classifications were abstracted from the delivery record and confirmed by the study obstetrician. Seventy-six (18%) participants were overweight and 58 (14%) were obese. A total of 49 (11.7%) preterm births were observed, consisting of 36 (73%) spontaneous and 13 (27%) medically indicated. After adjusting for pregnancy complications, previous preterm birth, age, acculturation, and gestational weight gain, obese teens had a reduced odds of total preterm birth (OR: 0.12, 95% CI: 0.02, 0.61) and had a mean gestational age at delivery of 0.9 weeks higher (95% CI: 0.19, 1.56) as compared to normal weight teens. When evaluating preterm birth by subtype, overweight/obese teens had a reduced odds of spontaneous (OR: 0.36, 95% CI: 0.13, 1.02) and medically indicated (OR: 0.054, 95% CI: 0.004, 0.70) preterm birth compared to normal weight teens. This study adds to the body of literature on the impact of obesity on birth outcomes and extends this work to Hispanic teenagers.