Louder, T, Bressel, E, Nardoni, C, and Dolny, D. Biomechanical comparison of loaded countermovement jumps performed on land and in water. J Strength Cond Res 33(1): 25-35, 2019-Researchers have observed physical improvements after the completion of aquatic-based jump training. However, there is a lack of research on the biomechanical specificity of aquatic-based movement. Therefore, the purpose of this investigation was to evaluate the kinetics and kinematics of loaded countermovement jumps performed in water versus land. Twenty young men and 24 National Collegiate Athletic Association (NCAA) Division I female soccer and gymnastics athletes were asked to perform unloaded and loaded countermovement jumps on land and in chest-deep water immersion. A triaxial force platform and 2-dimensional videography produced various kinetic and kinematic measures of jump performance. Peak and mean mechanical power outputs (W) were 88% (8,919 ± 3,744 vs. 4,734 ± 1,418 W; p < 0.001) and 81% (3,640 ± 1,807 vs. 2,011 ± 736 W; p < 0.001) greater for jumps performed in water vs. land. Peak dorsiflexion velocity was 688% faster (44 ± 39 vs. 5.6 ± 5.4 degree·s; p < 0.001) for jumps performed in water and tended to model similarly with measures of mechanical power and amortization rate. Body weight normalized peak and mean mechanical power outputs decreased by 23.6 ± 2.7 and 23.8 ± 1.9% when load was added in the water. The addition of load on land was associated with an 8.7 ± 2.3 and 10.5 ± 4.4% decrease in body weight normalized peak and mean mechanical power. Results suggest that the aquatic environment alters movement primarily at amortization and may provide a unique training stimulus. Also, it can be concluded that fluid resistance and buoyancy combine to influence the mechanics of jumping movements performed in the water.