Adult online gaming is a fast-growing global entertainment industry, and many gamers spend more time playing games and less time engaging in work or other activities, which negatively affects their lives and relationships. This quantitative study addressed how psychological needs and gaming motivation predicted gaming behavior, life satisfaction, and relationship satisfaction using a mediational model. The theoretical foundation was self-determination theory, which addresses how personal choices are influenced by internal factors, especially psychological needs and motivation. An online survey was used to collect self-reported data from a convenience sample of 935 adult gaming participants using the Basic Needs Satisfaction in General Scale, the Gaming Motivation Scale, the Satisfaction with Life Scale, the Relationship Assessment Scale, and a short demographic questionnaire. Results of multiple regression analyses indicated gaming motivation was a significant mediator of life satisfaction and relationship satisfaction. However, gaming motivation was not a significant mediator of gaming behavior. Findings of this study indicate that for online gamers, quality of life (life satisfaction and relationship satisfaction) is enhanced when gaming motivation is high, suggesting that joy of gaming can positively influence other aspects of life, when psychological needs (autonomy, competence, and relatedness) are positive as well.