Summary Background Previous studies concerning the effect of hamstring stretching and sacroiliac joint manipulation on hamstring flexibility or hip flexion range of motion have only concentrated on the immediate post-intervention effects. Objective To examine the effect of a 3-week trial of hamstring stretching compared to stretching and sacroiliac joint manipulation on hamstring flexibility as measured using the passive straight leg raise (SLR) and the back saver sit and reach (BSSR) test. Methods All 15 volunteers passed the inclusion criteria and had a passive straight leg raise less than or equal to 70°. The participants were randomly divided into either the hamstring stretching or stretching/manipulation group. Both protocols lasted for 3 weeks and involved participants stretching twice a day, being tested twice a week (SLR and BSSR tests) during which the manipulation group received bilateral sacroiliac joint manipulations. A final assessment was performed 1 week after the cessation of the protocol. Non-parametric Mann–Whitney U and Wilcoxen Signed Ranks tests were performed on the data. Results Both protocols produced significantly greater results for the SLR and BSSR tests. When compared, results for both tests in the manipulation group were greater than the stretch group. However, only increases in the passive straight leg raise on the left leg were significantly greater in the manipulation group. Conclusions The results suggest both hamstring stretching and stretching with sacroiliac joint manipulation significantly increase flexibility over a 4-week period; however, more research is needed to determine whether manipulation has a greater effect.