Background: Extracurricular school-based sports are considered to be an ideal means of reaching children who are not active in community sports. The purposes of this study were to examine the extent to which pupils not engaging in community sports do participate in extracurricular school-based sports, and to assess whether extracurricular school-based sports participants are more physically active and/or more autonomously motivated towards sports in daily life than children who do not participate in extracurricular school-based sports. Methods: One thousand forty-nine children (53.7% boys; M age = 11.02 years, SD = 0.02) out of 60 classes from 30 Flemish elementary schools, with an extracurricular school-based sports offer, completed validated questionnaires to assess physical activity (Flemish Physical Activity Questionnaire) and motivation (Behavioral Regulations in Physical Education Questionnaire). Multilevel regression analyses were conducted to examine the data generated from these questionnaires. Results: More than three quarters of the children (76%) reported participating in extracurricular school-based sports during the current school year and 73% reported engaging in organized community sports. Almost two third of the children (65%) not participating in community sports stated that they did participate in extracurricular school-based sports. Extracurricular school-based sports participants were significantly more physically active than children not participating in extracurricular school-based sports (β = 157.62, p < 0.001). Significant three-way interactions (sex × extracurricular school-based sports participation × community sports participation) were found for autonomous motivation, with boys engaging in extracurricular school-based sports but not in community sports being significantly more autonomously motivated towards sports than boys not engaging in community or extracurricular school-based sports (β = 0.58, p = 0.003). Such differences were not noted among girls. Conclusions: If extracurricular school-based sports are offered at school, the vast majority of elementary school children participate. Although extracurricular school-based sports attract many children already engaging in community sports, they also reach almost two third of the children who do not participate in community sports but who might also be optimally motivated towards sports. As children participating in extracurricular school-based sports are more physically active than children who do not participate, extracurricular school-based sports participation can be considered to contribute to an active lifestyle for these participating children.