Background Clamshell exercise is commonly used as a rehabilitation strengthening exercise for the gluteus medius muscle, which plays a large role in human walking, weight-bearing activities, and pelvic stabilization. Compensatory movements such as excessive horizontal rotation of the pelvis may occur during clamshell exercises. Various studies on pelvic control have been conducted, but studies on how to control pelvic rotation using tools are lacking. Objectives The purpose of this study was to investigate the effects on gluteus medius activity and pelvic rotation when adjusting pelvic rotation using a smartphone-based measurement tool during clamshell exercise. Methods This study recruited 27 healthy adult females. During clamshell exercise, gluteus medius muscle activity and pelvic rotation were investigated with and without the use of a smartphone-based measurement tool. Gluteus medius activity was measured with 4D-MT 2EM, and pelvic rotation was analyzed using the Kinovea® program. Statistical analysis was performed using t-test. Results The activity of the gluteus medius muscle significantly increased and the amount of pelvic rotation significantly decreased in the clamshell exercise with the use of the smartphone-based measurement tool compared to the clamshell exercise using the pelvic rotation control method without the use of a smartphone-based measurement tool (p < 0.001). Conclusions The use of a smartphone-based measurement tool during clamshell exercise can help the efficient activation of the gluteus medius muscle by minimizing compensatory actions such as unwanted rotation of the pelvis.