African Americans are less likely than other racial groups to engage in and complete outpatient substance abuse treatment. The current study, conducted as a secondary analysis of a multisite randomized clinical trial, examined whether readiness to change (RTC) over time influences retention and whether gender moderates the relationship between changes in RTC and retention among 194 African American women and men. Participants completed the University of Rhode Island Change Assessment at baseline and at the end of the 16-week study. Findings revealed a significant relationship between RTC over time and retention. Specifically, the more RTC increased throughout the 16-week study, the longer participants remained in treatment. In addition, gender moderated the relationship between changes in RTC and retention, with a stronger association between changes in RTC and retention among men relative to women. One approach to improving substance abuse treatment retention rates is to focus on increasing RTC during treatment, especially among African American men.