Background and Objectives: Low back pain is one of the most common complaints in pregnancy which interferes with work, daily activities and sleep. The aim of this study was to determine the effect of Sitting Pelvic Tilt Exercise on physical disability in primigravidas with low back pain during the third trimester. Materials and Methods: This quasi-experimental study was carried out on 60 primigravidas who attended in 3 prenatal clinics of Zanjan university of medical science. At first, 60 primigravidas were selected by simple sampling and randomly divided into two groups: case and control. Only the experimental group did the sitting pelvic tilt exercise program for 8 weeks during the third trimester. Data was collected for both groups by interview using VAS scale and Disability Rating Index (DRI) on the days 0 and 56 (pre and post intervention). Results: There was no significant difference between the two groups in terms of age, occupation, education and BMI and intensity of physical disability before the intervention. However, intensity of physical disability significantly decreased (p<0.0001) in the experimental group after the exercise program. Adversely, the intensity of physical disability increased in the control group as pregnancy proceed (p<0.0001). Intensity of low back pain was also reduced in the experimental group after the sitting pelvic tilt exercise program comparing to pre treatment stage and also in comparison with the control group (P<0.0001). Conclusion: It seems that use of exercise programs such as “sitting pelvic tilt exercise” is an effective method in pregnant women who suffer from low back pain and related disabilities but do not want to use aggressive treatments and chemical pain relief drugs.