Loneliness is increasingly recognized as a critical public health concern, with growing evidence of the effectiveness of interventions to reduce loneliness across the lifespan. This preregistered systematic review and meta-analysis evaluated the effectiveness of interventions to reduce loneliness. The systematic review identified 312 studies. The meta-analysis included 280 studies (273 studies of short-term effects; 72 studies of long-term effects). Using random-effects models, a small to moderate short-term effect on loneliness (up to 4 weeks after the intervention) was observed (122 randomized controlled trials: standardized mean difference = -0.50, 95% confidence interval [-0.60, -0.39]; 33 multicohort studies: standardized mean difference = -0.51, 95% confidence interval [-0.68, -0.34]; 118 single-arm cohort studies: standardized mean difference = -0.38, 95% confidence interval [-0.46, -0.30]). Confidence in the estimates was assessed using the Grading of Recommendations, Assessment, Development, and Evaluation system and graded as low or very low. No statistically significant differences were found between age groups. Psychological interventions appeared to be the most effective intervention strategy for reducing loneliness, demonstrating a moderate effect, while social and emotional skills training, social network interventions, and social support interventions showed small to moderate effects. Further analyses demonstrated that long-term effects (1-6 months after the intervention) were comparable to short-term effects. The current meta-analysis provides overall evidence of the effectiveness of loneliness interventions. Given methodological limitations, it remains unclear whom the interventions would help the most. Overall, there is a need for rigorous and high-quality development and further evaluation of interventions for loneliness. (PsycInfo Database Record (c) 2026 APA, all rights reserved).