Online gaming is an appealing and popular activity among adolescents. However, several studies have shown that excessive engagement is related to psychosocial problems. The present study is based on data from the EU NET ADB project and examined the association between problematic online gaming and psychosocial wellbeing in a Greek school-student’s sample. Problematic online gaming is classified in the DSM-5 as Internet Gaming Disorder (IGD) and was measured with the Assessment of Internet and Computer game Addiction Scale — Gaming Module (AICA-S-gaming). Negative aspects of psychosocial well-being were operationally defined as a combination of behavioral, affective and cognitive problems and were assessed through the Youth Self-Report (YSR) scale. The results indicate that IGD has a significant impact on the externalizing and internalizing dimensions of the YSR scale and particularly in the subscales anxious-depressed, withdrawn-depressed, social problems, thought problems, somatic complaints, rule-breaking behavior and aggressive behavior. Limitations and applications of the study are discussed.