Background: Videogame addiction has been suggested as a tentative disorder in 2013 by the American Psychiatric Association (APA) and was recently officially recognized as a mental health disorder by the World Health Organization (WHO). Although a few studies have identified attention deficit and hyperactivity disorder (ADHD) as a key risk factor for Internet Gaming Disorder (IGD), the interplay between ADHD and IGD symptoms with gender differences across cultures remains to be further examined. Objective: This study examined the moderating effects of gender in the association between ADHD and IGD across two nations. Method: A cross-sectional online survey was developed to recruit 164 Australian (Mage = 23.01, SD = 3.35, Minage = 18, Maxage = 31, Males n = 121, 73.80%) and 457 U.S.-North American (Mage = 25.25 years, SD = 2.76, Minage = 18 years, Maxage = 29 years, Males = 265, 57.98%) Massively Multiplayer Online (MMO) players aged between 18 and 29 years. Results: The hierarchical linear regression, moderation and moderated moderation analyses revealed that participants presenting greater inattention and hyperactivity symptoms exhibited higher levels of IGD-related behaviors in the two samples. Moreover, these associations differed across genders between the two countries. Specifically, more hyperactive-impulsive, as well as inattentive males in the USA presented higher levels of disordered gaming. Conclusion: The results highlight the need for more cross-cultural and symptom-focused research in the broader IGD field.