Mexico occupies the first places in obesity worldwide, presenting high levels of this condition and low levels of physical activity. For this reason, the study of adoption and maintenance towards weight control from a psychological approach has a significant relevance in the Mexican context. Within the framework of grit personality, the self-determination theory and the model of the stages of change, this study evaluated the effect of Grit over the motivation towards weight control and its relationship with the stages of change in an adult population in the state of Nuevo Leon. 1351 adults participated in the study (Mage = 29.59; SD = 12.01), 50.9% (n = 688) were males and 49.1% (n = 663) females, using a multistage sampling. The instruments used were the Grit personality scale, the behavioral regulation in exercise questionnaire, and the stages of change questionnaire (the last two were adapted to weight control). Quantitative methods were used, with a descriptive, correlational, cross-sectional, field and non-experimental study design. Exploratory and confirmatory factor analysis, reliability analysis, descriptive statistics, multivariate analysis, Pearson correlations, and structural equation modeling were performed using IBM SPSS statistics 23 and LISREL 8.80 software. The data collection was carried out in Monterrey, and the participants responded to the instruments in a self-administered manner with the supervision of interviewers. The most relevant results of this study focused on the hypothesized model on the relationship between Grit, types of motivation and stages of change in weight control, which showed a good fit to data (2 = 7554.199, df = 1521; p < .001; /df = 4.96; NNFI = .92; CFI = .94; RMSEA = .06 (90% CI = .0637;.0667); AIC model = 7818.199; SRMR = .12). Concretely, results indicated a positive relationship between Grit and autonomous motivation, and a negative relationship with controlled motivation and amotivation. Moreover, autonomous motivation showed a positive relationship with the stages of maintenance, action, preparation and contemplation; and a negative relationship with precontemplation stage. Controlled motivation was positively related to the five stages of change; and amotivation was negatively related to the stages of preparation and contemplation, and positively related to the pre-contemplation stage. In addition, Grit showed significant indirect effects on the stages of change through the types of motivation. Specifically, a positive indirect effect was found with the maintenance, action, preparation and contemplation stages, and a negative indirect effect with the precontemplation stage. Finally, Grit explained 40% of the variance of the maintenance and action stages, 30% of the variance of the preparation stage, 56% of the variance of the contemplation stage, and 31% of the variance of the precontemplation stage. The results confirm the theoretical approach of this research, giving Grit a key role in the change and maintenance of healthy behaviors specifically in weight control.