Currently, there is no screening instrument available for assessing both internet gaming disorder (IGD) and social media disorder (SMD). This study aimed to examine the reliability and factor structure of a new screening instrument for adolescents and adults, the Gaming and Social Media Questionnaire (GSMQ-9), and to investigate its association with psychosocial outcomes (i.e., psychosomatic problems, self-concept, and social problems for adults and quality of life for adolescents). Survey data were collected from 995 university students and 626 adolescents. Results showed that a two-factor solution, representing Heavy Involvement and Negative Consequences, had a better model fit compared to a one-factor solution for both IGD and SMD and for both adolescents and adults. The internal consistency was acceptable, and the test-retest reliability was excellent. Negative Consequences were significantly more strongly related to all psychosocial outcomes compared to Heavy Involvement. The proportion of participants meeting the DSM-5 symptom criteria according to self-ratings on the GSMQ-9 was 1.4% (adolescents) and 1.8% (adults) for IGD and 2.6% (adolescents) and 4.0% (adults) for SMD. Conclusively, the GSMQ-9 appears to be a reliable two-factor screening instrument for IGD and SMD among adults and adolescents.