Simple Summary: There is increasing interest in the effect of saddle design on horse kinematics, but little evidence of the influence on rider–saddle interaction and how this affects horse movement patterns. We aimed to investigate the effect of changing the design of the saddle’s thigh block on the interaction between the rider and saddle and the effect this has on rider movement and horse movement. To do this, we used a seat pressure mat between the rider and the saddle and tracking technology to analyse horse and rider movement. Elite level sports horses, ridden by elite level riders, were trotted in well-fitting dressage saddles that were identical, except for the thigh block design. During straight-line locomotion when in sitting trot, results showed that a thigh block with a more deformable face (thigh block F) resulted in a greater contact area and more pressure between the rider’s seat and the saddle as well as a more upright rider position when the horse’s limbs were on the ground. An association between thigh block design, horse spinal movement, and forelimb flexion was also seen. These findings illustrated the importance of optimizing rider–saddle–horse interaction. Abstract: The association between rider–saddle interaction and horse kinematics has been little studied. It was hypothesized that differences in a thigh block design would influence (a) rider–saddle interface pressures, (b) rider kinematics, and (c) equine limb/spinal kinematics. Eighteen elite sport horses/riders were trotted using correctly fitted dressage saddles with thigh blocks S (vertical face) and F (deformable face). Contact area, mean, and peak pressure between rider and saddle were determined using an on-saddle pressure mat. Spherical markers allowed for the measurement of horse/rider kinematics using two-dimensional video analysis. The kinematics of the equine thoracolumbosacral spine were obtained using skin-mounted inertial measuring units. Results were compared between thigh blocks (paired t-test p ≤ 0.05). With F, the contact area, mean, and peak pressure between rider and saddle were significantly higher (p = 0.0001), and the rider trunk anterior tilt was reduced, indicating altered rider–saddle interaction. The horse thoracic axial rotation and flexion/extension were reduced (p = 0.01–0.03), caudal thoracic and lumbar lateral bend was increased (p = 0.02–0.04), and carpal flexion increased (p = 0.01–0.05) with F compared to S. During straight-line locomotion when in sitting trot, thigh block F was associated with altered rider–saddle interaction and rider and equine kinematics, leading to a more consistent rider–saddle interface, a more upright rider trunk during stance, an increased horse thoracic stability and lumbar lateral bend, and forelimb flexion, supporting the importance of optimising rider–saddle–horse interaction.