In order to achieve success, competitive athletes are required to meet the extensive demands and overcome the numerous obstacles that are associated with their sporting careers. The construct of grit—characterized by passion and perseverance over extended periods of time—has been associated with goal achievement in sport psychology research. However, the advancement of this field has been hindered by a lack of sufficient theory. The purpose of this study was to construct a grounded theory of competitive athletes’ grit in sport using constructivist grounded theory methodology. Twenty-eight adult participants (15 women, 13 men; Mage = 27.3 years, SD = 8.2; 22 athletes, 5 coaches, 1 sport parent) involved in competitive sport participated in one-on-one semi-structured interviews. In total, 1933 minutes of audio was recorded. Data analysis followed an iterative process of initial coding, focused coding, axial coding, and theoretical integration. The constructed grounded theory suggests that grit was conceptualized as a malleable dispositional tendency formed over time as athletes amassed various positive and setback experiences in sport. With the encouragement of supportive others, athletes would adopt adaptive cognitions about success and failure in sport. These cognitions would then develop into a propensity to identify and strive towards personally meaningful long-term goals in sport. Grit was understood to lead to several outcomes; including sport-specific goal achievement, athlete thriving, and athlete languishing. This study contributes to the broader understanding of the underlying mechanisms that encapsulate competitive athletes’ sport-specific grit and offers implications for practitioners and sport researchers.