Positive psychological interventions (PPIs), which aim to cultivate psychological well-being, have the potential to improve health behavior adherence. This systematic review summarized the existing literature on PPI studies with a health behavior outcome to examine study methodology, quality, and efficacy. Of the 27 identified studies, 20 measured physical activity, eight measured medication adherence, seven measured diet, and three measured smoking (eight targeted multiple behaviors). Twenty studies were randomized controlled trials (RCTs; 13 fully powered), and seven had a single-arm design. Study samples were usually adults (n = 21), majority non-Hispanic white (n = 15) and female (n = 14), and with a specific disease (e.g., diabetes, n = 16). Most interventions combined a PPI with health behavior-focused content (n = 17), used a remote delivery method (n = 17), and received a moderate or low study quality rating. Overall, 19/27 studies found a health behavior improvement of at least medium effect size, while six of the 13 studies powered to detect significant effects were statistically significant. Of the behaviors measured, physical activity was most likely to improve (14/20 studies). In summary, PPIs are being increasingly studied as a strategy to enhance health behavior adherence. The existing literature is limited by small sample size, low study quality and inconsistent intervention content and outcome measurement. Future research should establish the most effective components of PPIs that can be tailored to different populations, use objective health behavior measurement, and robustly examine the effects of PPIs on health behaviors in fully powered RCTs.