The aim of this study was to investigate the effect of positive motivational self-talk on mental toughness, self-esteem, and sport self-efficacy among male student-athletes in the first year of secondary school in Sabzevar during the 2023–2024 academic year. This research was conducted using a quasi-experimental approach with a pre-test–post-test control group design. The statistical population consisted of all male student-athletes in the first year of secondary school in Sabzevar. A sample of 30 volunteer male student-athletes from a local football school was randomly selected and assigned to two groups: experimental (15 participants) and control (15 participants). The experimental group received six 90-minute sessions (three sessions per week) of positive motivational self-talk training, while the control group remained on a waiting list. Data collection tools included the Mental Toughness Questionnaire by Sheard and Golby (2009), the Rosenberg Self-Esteem Scale (1965), and the Sport Self-Efficacy Scale by Sahrayian, Vaezi-Mousavi, and Khabiri (2016). The collected data were analyzed using descriptive statistics (mean and standard deviation) and univariate and multivariate analysis of covariance (ANCOVA) via SPSS version 24. The results indicated a positive and significant effect of positive motivational self-talk on mental toughness, self-esteem, and sport self-efficacy in the male student-athletes (P < 0.05). Specifically, in the domain of sport self-efficacy, 40% of the variance in adjusted post-test scores between the groups was explained by the motivational self-talk intervention.