Internet-based weight loss programs can be effective in promoting weight loss and are less-intensive than traditional face-to-face approaches, which may provide more flexibility for postpartum, low-income women to engage in such programs. Few studies have examined patterns of engagement in internet-based programs for this population. This article used data from the internet-based Fit Moms/"Mamás Activas" intervention, a 12-month cluster randomized controlled trial that was effective in promoting postpartum weight loss among low-income, predominantly Hispanic women. The overall objectives of this study were to (1) characterize patterns of engagement with the Fit Moms/"Mamás Activas" website among intervention participants and (2) explore associations between engagement and 12-month weight loss outcomes among study completers (87.4%). A number of engagement variables were calculated for each participant, including website logins; time spent on the website; number of posts to the "Discussion Forum;" number of days tracking weight, diet, and physical activity; number of page visits to various website components; and number of in-person visits attended. The average number of logins was 70.74 (approximately once weekly), and average total time spent on the website was 185.35 minutes (approximately 3 hours) over 1 year. Self-monitoring ("Web Diary") and social support ("Discussion Forum") were the most frequently visited components of the website, and more frequent engagement with these components, as well as greater attendance at in-person group sessions, predicted greater percent weight loss at 12 months. Interventions highlighting these features may be particularly effective for weight loss in this population.