Weight-related self-monitoring (WRSM) apps are often used by emerging adults to assist in behavior change. However, little is known about the relationship between WRSM among the general population of emerging adults and various physical activity and screen time behaviors. This paper examines associations between WRSM app use and various forms of physical activity and screen time among a population-based sample of emerging adults. Data come from EAT 2018 (N = 1,568, mean age = 22.0 ± 2.0 years), a population-based sample of emerging adults from Minneapolis/St. Paul, Minnesota. Participants reported on the types of WRSM apps used (physical activity and/or dietary focused), in addition to how much time they spent doing physical activity, whether they practiced yoga, compulsive exercise, recreational screen time, social media use, and whether they viewed dieting/weight loss materials. Linear and logistic regressions were used to assess adjusted means and prevalences of outcomes. We found that physical activity-focused WRSM app users engaged in more hours of total (8.7 vs 7.2,  < .001), and moderate-to-vigorous (5.1 vs 4.3,  = .002) physical activity compared to non-users. Similar results were found for dietary WRSM app users compared to non-users. However, WRSM app users had higher levels of compulsive exercise and were more likely to view dieting/weight loss materials ( < .001). Findings suggest that although physical activity is higher among WRSM users, the types of physical activity and screen time behaviors WRSM users are engaging in may be harmful.