Although researchers have previously discussed the consequences of extracurricular activity (ECA) participation, the question of which factors affect the intention to participate in ECAs has been less addressed. By incorporating the theory of planned behavior (TPB) and signaling theory, this study investigates the role of the components of the TPB and signal quality in the intention to join ECAs. In addition, the contingent effects of students' demographic profiles, such as their gender, their year in school, and whether they have a part-time job, are examined. A sample of 386 students at various universities in Ho Chi Minh, Vietnam, was used to test a model that incorporates the relationships between TPB components and signal quality by means of the partial least squares (PLS) technique. The results of this study indicate that the three components of the TPB have positive impacts on the intention to join ECAs. In addition, signal quality affects both attitude and intention. Finally, students' demographic profiles, such as their gender, their year in school, and whether they have a part-time job, have moderating roles in the relationship from attitude and subjective norms to intention. This study adopted a cross-sectional design and was conducted in Vietnam only. Future research should consider longitudinal studies and make comparisons between countries with diverse cultures. As another limitation, this study investigated only university students. Future research, therefore, is needed to consider other subjects and different sectors, such as high school pupils. The results of this study suggest that ECA organizers should send clear, consistent, and credible signals to potential members. Additionally, parents, teachers and other influencers should talk to and encourage students to join ECAs. As a major contribution of this study, it incorporates the TPB and signaling theory to examine the antecedents of ECA participation.