Background: Mycoplasma genitalium is associated with cervicitis and pelvic inflammatory disease in nonpregnant women. We investigated associations between cervical M genitalium, demographic and behavioral risk factors for sexually transmitted infection and preterm birth among low-income Peruvian women. Methods: This case-control study, conducted at the Instituto Nacional Materno Perinatal, Lima, Peru, included 661 cases with a spontaneous preterm birth at <37 weeks and 667 controls who delivered at ≥37 weeks. Within 48 hours after delivery, subjects underwent interviews, medical record review, and collection of cervicovaginal specimens for M. genitalium, Chlamydia trachomatis, and Neisseria gonorrhoeae by nucleic acid amplification testing, and Trichomonas vaginalis by culture. Odds ratios and 95% confidence intervals were calculated for associations between M. genitalium, other genital infections and risk factors, and preterm birth. Multivariable logistic regression was used to adjust for potential confounders. Results: Cervical M. genitalium was detected in 3% of subjects and was significantly associated with C. trachomatis infection (P < 0.001) and preterm birth (4% vs. 2%; adjusted odds ratio: 2.5, 95% confidence interval: 1.2-5.0, P = 0.014), and marginally associated with T. vaginalis (P = 0.05). M. genitalium detection was also associated with younger maternal age (P = 0.003) but not with other risk factors for preterm birth. The association between cervical M. genitalium detection and preterm birth remained significant after adjustment for maternal age and coinfection with C. trachomatis or T. vaginalis. Conclusions: Cervical M. genitalium detection was independently associated with younger maternal age and preterm birth, suggesting that this organism may be an infectious correlate of spontaneous preterm birth. © Copyright 2010 American Sexually Transmitted Diseases Association.