Introduction: Postpartum relapse rates are high among women who spontaneously quit smoking during pregnancy. This randomized clinical trial tested a Motivation and ProblemSolving (MAPS) treatment for reducing postpartum relapse among diverse low-income women who quit smoking during pregnancy (N = 251; 32% Black, 30% Latino, and 36% White; 55% <$30,000/year household income).Methods: Pregnant women were randomly assigned to MAPS/ MAPS+ or Usual Care (UC). Continuation ratio logit models were used to examine differences in biochemically confirmed continuous abstinence at Weeks 8 and 26 postpartum by treatment group and moderators of the treatment effect. Analyses controlled for age, race/ethnicity, partner status, education, smoking rate, and the number of smokers in the participant's environment.Results: MAPS/MAPS+ was more efficacious than UC in the prevention of postpartum relapse (p = .05). An interaction between treatment and the number of cigarettes smoked per day before quitting approached significance (p = .09), suggesting that the MAPS/MAPS+ treatment effect was stronger among women who smoked more cigarettes per day.Discussion: MAPS, a holistic and dynamic approach to changing behavior using a combined motivational enhancement and social cognitive approach, is a promising intervention for postpartum smoking relapse prevention among low-income women, which may have particular relevance for women with higher prequit smoking rates.