The aims of the study were (a) to test the effectiveness of the theory of planned behavior (TPB) to predict Greek university students’ voluntary participation in an extracurricular gymnastics course, and (b) to evaluate gender differences. Two hundred sixty-three (127 female, 136 male) students participated in the study. Students’ attitudes, intention, and PBC were measured with a questionnaire and their attendance in the course was recorded by the teacher. Results from the MANOVA conducted showed that females had higher scores compared with males in all observed variables. Results from the structural equation modeling (SEM) employed supported the usefulness of TPB to explain students’ attitudes and behavior toward extracurricular physical activities (PA). Differences also emerged on path structure of the relationships among the variables.