Soft tissue mobilization techniques have been reported to increase hamstring flexibility. However, no one has reported the effects of the Graston Technique® on increasing hamstring flexibility. The purpose of this study was to determine the changes in range of motion after applying the Graston Technique® to tight hamstring muscles. Forty five college-aged students participated; 3 well-trained athletes, 35 physically trained students, and 7 untrained students (total 30 men and 15 women). Pre- and post-treatment measurements on hamstring flexibility were taken at the 90/90° position using a goniometer. There was a statistically significant difference between the sham treatment group and the treatment groups (an only Graston Technique® group and a whole Graston Technique® treatment group). However, there was no significant difference between the only Graston Technique® group and the whole Graston Technique® treatment group. Also, range of motion increase was affected by the adipose tissue thickness.