Adolescents spend a substantial part of their leisure time with playing games and using social media such as Facebook. The present paper examines the link between adolescents' computer and Internet activities and computer literacy (defined as the ability to work with a computer efficiently). A cross-sectional study with N = 200 adolescents, aged 17 on average, was conducted. Hierarchical regression analyses showed that an increase in time spent playing games on a PC/Mac was related to higher scores on practical and theoretical computer knowledge. Moreover, practical computer knowledge was higher for adolescents who liked playing shooters, fantasy games, or Facebook-games. Frequency of social media use was associated with higher scores in practical computer knowledge. A bootstrap analysis indicated that this relationship was mediated by a decrease in computer anxiety, not by more positive attitudes toward the computer. Gender yielded substantial main effects on the media literacy variables but the associations between the computer and Internet activities and the literacy aspects were similar for boys and girls. This study is expected to encourage future longitudinal research on adolescents' incidental learning during leisure time computer and Internet activities. (Contains 1 figure and 4 tables.)