Early specialization in sport has been linked to negative consequences, such as injury, burnout, and attrition. However, research has been hampered by the lack of a standardized definition and reliable measurement of sport specialization (Jayanthi et al., 2015). Recently, the American Orthopaedic Society for Sports Medicine reached consensus on a definition of early sport specialization comprising three criteria: 1) participation in intensive training and/or competition in organized sports for more than 8 months per year, 2) participation in one sport to the exclusion of participation in other sports, and 3) involving prepubertal children (around age 12 years) children (LaPrade et al., 2016). This study aimed to lay the groundwork for the development and validation of a measure of early specialization in sport, based on this new definition and other previously used measures. Secondary data analysis was conducted on retrospective-longitudinal survey data collected from 255 swimmers (Mage = 13.8 years; range = 12-17). Data included detailed descriptions of sport backgrounds, including season durations, frequency of weekly practices, and total weekly hours in each sport from age 6. Exploratory factor analyses determined how eight particular survey items represented data for swimming specialization, their communalities and loadings on a single factor, and measures of internal consistency reliability. We employed this single factor score to plot the developmental profile of swimmers from 6-17 years of age. We discuss the utility of such a measure, how it could be used in path analyses, and whether its operationalization is suitable based on evolving definitions of specialization.