Introduction: Assertive behavior in adolescence is important for well-being and socio-emotional functioning, therefore having valid instruments that are comparable across subgroups is essential. The Assertive Behavior Self-Report (ADCA-1) is frequently used, but its structure and equivalence across gender and age in adolescent populations require further evidence. Objective: To determine the psychometric properties and factorial invariance of the Assertive Behavior Self-Report (ADCA-1) in adolescents. Method: An instrumental design was used, with a non-probabilistic intentional sample consisting of 229 students aged 14 to 17 years (M = 15.44; SD = .82), 50.7% were male and 49.3% female. The instrument used was the Assertive Behavior Self-Report (ADCA-1). Results: Data were analyzed through a CFA for polychoric matrices and a WLSMV estimator, f...