Recent research has suggested that metacognitions may play a role across the spectrum of addictive behaviours. The goal of our studies was to develop the first self-report scale of metacognitions about gambling. We conducted three studies with one community (n = 165) and two clinical (n = 110; n = 87) samples to test the structure and psychometric properties of the Metacognitions about Gambling Questionnaire and examined its capacity to prospectively predict severity of gambling. Findings supported a two factor solution consisting of positive and negative metacognitions about gambling. Internal consistency, predictive and divergent validity were acceptable. All the factors of the Metacognitions about Gambling Questionnaire correlated positively with gambling severity. Regression analyses showed that negative metacognitions about gambling were significantly associated to gambling severity over and above negative affect and gambling-specific cognitive distortions. Finally only gambling severity and negative metacognitions about gambling were significant prospective predictors of gambling severity as measured three months later. The Metacognitions about Gambling Questionnaire was shown to possess good psychometric properties, as well as predictive and divergent validity within the populations that were tested.