Stiffness of the posterior deltoid is as a causative factor in the limited range of glenohumeral horizontal adduction and various other shoulder pathologies including shoulder impingement syndrome, frozen shoulder, and humerus anterior glide syndrome. The purpose of this study was to compare the effects of two techniques (soft tissue massage and cross-body stretch) on increasing the range of horizontal adduction. Thirty-two subjects with a $10^{circ}$ or greater difference between the right and left sides in horizontal adduction were selected. Sixteen subjects from each group were allocated randomly. Interventions were applied on six occasions for 2 weeks, and the range of horizontal adduction was measured using an inclinometer at pre-and post-intervention. A $2{ imes}2$ analysis of variance (intervention${ imes}$time) was used to compare the effects of the two techniques. In the soft tissue massage group, the angle of horizontal adduction significantly increased compared with the cross-body stretch group. These findings indicate that the soft tissue massage of the posterior deltoid muscle is a more effective method to increase the flexibility of the glenohumeral horizontal adduction.