Many students have low reading motivation. Based on (reading) motivation theories, several mechanisms are distinguished that can foster reading motivation. Our goal in this meta-analysis was to examine the effects of theory-driven reading motivation interventions in school on students’ reading motivation and reading comprehension as well as to test which mechanisms are particularly effective in fostering motivation and comprehension. We conducted a literature search in ten online databases and identified 39 relevant effect studies. Positive effects on affirming motivations (d = 0.38), extrinsic motivations (d = 0.42), combined motivations (d = 0.17), and reading comprehension (d = 0.27) were found. The effect on undermining motivations (d = −0.01) was not significant. In particular, interventions that aimed to trigger interest had positive effects on affirming motivations and reading comprehension. Furthermore, effects on affirming motivations were larger if the total duration of the intervention was longer and if the share of boys in the sample was higher. Interventions delivered by researchers had larger effects on reading comprehension than interventions delivered by teachers. Finally, effects on reading comprehension were larger for primary schoolers than for secondary schoolers and larger for typical readers than for struggling readers. Implications for practitioners, policymakers, and researchers are discussed.