This study aimed to adapt and analyze the psychometric properties of the Exercise Behavior Regulation Questionnaire (BREQ-3) for assessing motivation towards incidental physical activity. An instrumental study in a sample of 346 university students (21.1 +/- 2.6 years, and 61.3% women) from various universities in Chile was undertaken. An adaptation of the BREQ-3 was applied, and a confirmatory factor analysis was performed using a robust weighted least squares estimator to assess the construct validity of the scale. Also, the convergent validity was evaluated using the average variance extracted, the discriminant validity using composite reliability, and the internal consistency using Cronbach's alpha (alpha) and McDonald's omega (omega) coefficients. The six-factor structure of intrinsic motivation (alpha = 0.96, omega = 0.96), integrated regulation (alpha = 0.95, omega = 0.95), identified regulation (alpha = 0.89, omega = 0.90), introjected regulation (alpha = 0.75, omega = 0.77), external regulation (alpha = 0.80, omega = 0.83), and amotivation (alpha = 0.75, omega = 0.79), with acceptable fit indices after eliminating items 8 and 11, was confirmed-chi 2/df: 2.196, CFI: 0.99, TLI: 0.99, RMSEA: 0.059 (90% CI; 0.051-0.067). Adaptation of the BREQ-3 appears to be a reliable measure for assessing motivation in the context of incidental physical activity. Its use will contribute to understanding the explanatory mechanisms underlying this behavior.