 This study aimed to identify distinct groups of smartphone app login trajectories over a six-month period for two smoking cessation apps, their association with smoking cessation outcomes at 12 months, and baseline user characteristics that predict data-driven trajectory group membership. The results showed that patterns of one dash, three dash, and four-week smartphone app use for smoking cessation may be common in how people engage in digital health interventions. There were significantly higher odds of quitting smoking among four-week users and especially among 26-week users of the iCanQuit app. To improve study outcomes, strategies for detecting users who disengage early from these interventions, one-week users, and proactively offering them a more intensive intervention could be fruitful. This article was authored by Jonathan B. Bricker, Kristin E. Moll, Margarita Santiago Torres, and others.