Limb girdle rotations can make important contributions to the locomotor performance of tetrapods by facilitating increases in stride length. Most previous studies of girdle rotations have focused on hindlimb function during terrestrial locomotion. However, the forelimb also has a critical locomotor role among tetrapods in water and on land, with a different means of attachment to the body that might impact its capacity for rotation. The pelvis functions as a single, fused element that articulates closely with sacro-iliac joints, whereas the left and right pectoral girdles are attached to the body via a muscular sling and a flexible cartilaginous attachment, and have the potential to move independent of one another. As a result, pectoral girdle rotations may actually be greater than pelvic girdle rotations. To test for such differences, we used X-Ray Reconstruction of Moving Morphology (XROMM) to compare pectoral and pelvic girdle rotation between walking and swimming in the semiaquatic turtle Pseudemys concinna. Both girdles rotated in both environments. As predicted based on differences in girdle morphology, the shoulder girdle rotated more than the pelvic girdle during both walking (forelimb ~ 39°, hindlimb ~ 18°) and swimming (forelimb ~ 36°, hindlimb ~ 9°). However, the effect of the medium on shoulder girdle movements was small, whereas pelvic girdle rotations during swimming were half the amount during walking. This suggests that body support plays a large role in determining the impact of pelvic girdle rotations, whereas muscular contraction may determine pectoral girdle rotations.