Mobile gaming has gained popularity among adolescents, and an increase in problematic use has been reported. The aims of this study are as follows: (a) develop a self-report questionnaire, the Problematic Mobile Gaming Questionnaire (PMGQ); (b) establish a validated cutoff value using structured interviews; and (c) evaluate the prevalence of mobile gaming addiction in adolescents. The PMGQ was built as a 12-item questionnaire rated on a 4-point Likert scale to evaluate the symptoms of problematic mobile gaming (PMG). The construct validity of the PMGQ was examined using exploratory factor analysis. Overall, 10,775 students with smartphones from grade 4 to senior high school were recruited to complete the questionnaire. A total of 113 senior high school students were interviewed using previously developed criteria for PMG to develop an optimal cutoff point measuring sensitivity, specificity, and diagnostic accuracy. The cutoff point was determined using the Youden index and optimal diagnostic accuracy. The PMGQ showed good internal consistency (Cronbach's α = 0.92) and adequate diagnostic efficiency (area under the receiver operating characteristic curve = 0.802). The items revealed three factors of addiction: compulsion, tolerance, and withdrawal. For the PMGQ, a cutoff point of 29/30 demonstrated the most optimal Youden index and diagnostic accuracy. Demographic data showed that the proportion of PMG was 19.1 percent among elementary school students, 20.5 percent among junior high school students, and 19.0 percent among senior high school students. The PMGQ demonstrated appropriate validity and accuracy in the assessment for PMG.