Three studies were conducted in order to develop and validate a mental toughness instrument for use in military training environments. Study 1 (n = 435) focused on item generation and testing the structural integrity of the Military Training Mental Toughness Inventory (MTMTI). The measure assessed ability to maintain optimal performance under pressure from a range of different stressors experienced by recruits during infantry basic training. Study 2 (n = 104) examined the concurrent validity, predictive validity, and test-retest reliability of the measure. Study 3 (n = 106) confirmed the predictive validity of the measure with a sample of more specialized infantry recruits. Overall, the military training mental toughness inventory demonstrated sound psychometric properties and structural validity. Furthermore, it was found to possess good test-retest reliability, concurrent validity, and predicted performance in two different training contexts with two separate samples