Aims: To measure the effect of cognitive-behavioral techniques (CBTs) on gambling disorder severity and gambling behavior at post-treatment and follow-up.Method: Seven databases and two clinical trial registries were searched to identify peerreviewed studies and unpublished studies of randomized controlled trials. The Cochrane Risk of Bias tool assessed risk of bias in the included studies. A random effect metaanalysis with robust variance estimation was conducted to measure the effect of CBTs relative to minimally treated or no treatment control groups. Results: Twenty-nine studies representing 3991 participants were identified. CBTs significantly reduced gambling disorder severity (g = −1.14, 95% CI = −1.68, −0.60, 95% prediction interval [PI] = −2.97, 0.69), gambling frequency (g = −0.54, 95% CI = −0.80, −0.27, 95% PI = −1.48, 0.40) and gambling intensity (g = −0.32, 95% CI = −0.51, −0.13, 95% PI = −0.76, 0.12) at post-treatment relative to control. CBTs had no significant effect on follow-up outcomes. Analyses supported the presence of publication bias and high heterogeneity in effect size estimates. Conclusions: Cognitive-behavioral techniques are a promising treatment for reducing gambling disorder and gambling behavior; however, the effect of cognitive-behavioral techniques on gambling disorder severity and gambling frequency and intensity at posttreatment is overestimated, and cognitive-behavioral techniques may not be reliably efficacious for all individuals seeking treatment for problem gambling and gambling disorder.