Establish whether digital self‐monitoring of diet and physical activity is effective at supporting weight loss, increasing physical activity and improving eating behavior in adults with obesity or overweight, and determine the intervention components that might explain variations in its effectiveness. A systematic search of MEDLINE, Embase, PsycINFO, Web of Science, Scopus, Cinahl, and CENTRAL identified 4068 studies, of which 12 randomized controlled trials were eligible and included in the review. A random‐effect meta‐analysis evaluated intervention effectiveness and subgroup analyses tested for effective intervention content. Twelve studies were included in the review and meta‐analysis. Digital self‐monitoring of both diet and physical activity had a statistically significant effect at supporting weight loss (mean difference [MD] = ‐2.87 [95% CI −3.78, −1.96], P < 0.001, I² = 69%), improving moderate physical activity (standardized mean difference [SMD] = 0.44 [95% CI 0.26, 0.62], P < 0.001, I² = 0%), and reducing calorie intake (MD = −181.71 [95% CI −304.72, −58.70], P < 0.01, I² = 0%). Tailored interventions were significantly more effective than nontailored interventions (x² = 12.92, P < 0.001). Digital self‐monitoring of physical activity and diet is an effective intervention to support weight loss in adults with obesity or overweight. This effect is significantly associated with tailored advice. Future studies should use rigorous designs to explore intervention effectiveness to support weight loss as an adjunct to weight management services.