Study Design Cross-sectional study. Background The Hip Stability Isometric Test (HipSIT) evaluates the strength of the hip posterolateral stabilizers in a position that favors greater activation of the gluteus maximus and gluteus medius and lower activation of the tensor fascia lata. Objectives To check the validity and reliability of the HipSIT and to evaluate the HipSIT in women with patellofemoral pain (PFP). Methods The HipSIT was evaluated with a handheld dynamometer. During testing, the participants were sidelying, with their legs positioned at 45° of hip flexion and 90° of knee flexion. Participants were instructed to raise the knee of the upper leg while keeping the upper and lower heels in contact. To establish reliability and validity, 49 women were tested with the HipSIT by 2 different evaluators on day 1, and then again 7 days later. The strength of the hip extensors, abductors, and external rotators was also evaluated. Twenty women with unilateral PFP were also evaluated. Results The HipSIT has excellent intrarater and interrater reliability. The standard error of measurement was 0.01 kgf/kg, and the minimal detectable change was 0.036 kgf/kg. The HipSIT showed good validity in isolated hip abduction, external rotation, and extension (P<.01). Women with PFP showed a 10% deficit in the HipSIT results for the symptomatic limb (P = .01). Conclusion The HipSIT showed excellent interrater and intrarater reliability, moderate to good validity in women, and was able to identify strength deficits in women with PFP. J Orthop Sports Phys Ther 2017;47(12):906-913. Epub 9 Oct 2017. doi:10.2519/jospt.2017.7274.