The aim of this study was to assess a potential relationship between indoor radon exposure and the incidence of lung cancer worldwide. A systematic literature search was carried out in PubMed, Web of Science, and Google Scholar to identify relevant studies published in English conducted in the last 15 years until January 2016. Summary relative risks (RR) and the corresponding 95% confidence intervals (CIs) were calculated using a random-effects model and the influence of moderators using a mixed-effects model. Heterogeneity was assessed using the Q, I 2 and H 2 tests, and the source of heterogeneity was detected by meta-regression analysis. Publication bias was evaluated with Egger’s regression symmetry test and the contour-enhanced funnel plot. Leave-one-out sensitivity analysis was performed. Twenty-five lung cancer studies (case-control studies) with 13,569 cases and 22,701 controls were included. Indoor Radon exposure was significantly associated with increased risk for lung cancer (RR, 1.19; 95% CI, 1.02–1.39). Study location analysis showed that radon exposure was associated with increased risk for lung cancer from forty degrees absolute latitude (RR, 1.09; 95% CI, 0.92–1.31), to fifty degrees (RR 1.26; 95% CI, 1.08–1.48), to sixty degrees (RR, 1.46; 95% CI, 1.12–1.91). Indoor radon exposure may be associated with increased risk for lung cancer.