Although previous research has shown that experiencing an injury can act as a catalyst for self-development, research that has examined the concept of sport injury-related growth (SIRG) remains largely descriptive. This study aimed to address this by developing a substantive theory to explain the processes through which injured athletes experienced SIRG. Using Strauss and Corbin’s (1998) variant of grounded theory, 37 injured athletes competing in a range of sports and competitive levels participated in qualitative interviews. Interviews (N = 70) and data analysis were conducted over a period of 24 months. Transcripts were analyzed using open, axial, and selective coding. Quality criteria used were fit, relevance, workability, and modifiability. The grounded theory produced (i.e., Theory of Sport Injury-Related Growth) suggests a number of internal (i.e., personality, coping styles, knowledge, and prior experience, and perceived social support) and external factors (i.e., cultural scripts, physical resources, time, and received social support) enable injured athletes to transform their injury into an opportunity for growth and development. The mechanisms through which this occurs are metacognitions, positive reappraisal, positive emotions, and facilitative responses. This theory offers a number of exciting avenues for future research, and provides medical personnel and practicing sport psychologists with a detailed explanation of how sport injury can lead to growth experiences.