© 2018 by the American College of Sports Medicine. Purpose This study aimed to assess the associations of prepregnancy cardiorespiratory fitness, moderate- to vigorous-intensity physical activity (MVPA), and time spent watching television with subsequent development of gestational diabetes mellitus (GDM). Methods Participants were 1333 women enrolled in the Coronary Artery Risk Development in Young Adults study who did not have diabetes either at baseline (1985-86) or before births occurring after baseline. Baseline fitness was estimated using a graded symptom-limited maximal exercise treadmill test and expressed in MET units. Baseline MVPA (exercise units per day) was measured using the Coronary Artery Risk Development in Young Adults physical activity history questionnaire, and television viewing (h·d-1) was assessed by self-report in 1990-1991. Logistic regression analysis was used to derive odds ratios and 95% confidence intervals, adjusting for time from baseline to delivery and baseline study center, age, race, education, parity, family history of diabetes, smoking, alcohol, saturated fat intake, waist circumference, homeostasis model assessment of insulin resistance, and HDL cholesterol. Results Over 25 yr of follow-up, 164 women developed GDM. The odds of developing GDM were 21% lower for each 1 SD increment in baseline level of fitness (2.3 METs, odds ratio = 0.79, 95% confidence interval = 0.65-0.96). Prepregnancy MVPA and television viewing were not statistically associated with the development of GDM. Conclusion Study findings indicate that objectively assessed prepregnancy fitness, but not self-reported MVPA or television time, is associated with GDM. Clinicians should counsel women on the benefits of improving fitness in the preconception period, particularly among women at greater risk for GDM.