Purpose: This study was aimed to determine the effects of thoracic spine thrust manipulation on muscle activities of the scapular upward rotators and middle deltoid, active range of motion (AROM), shoulder pain, and rounded shoulder posture in young adults with rounded shoulder. Methods: The subjects were 30 young adults (14 males, 16 females) with rounded shoulder. Thirty subjects were randomly assigned to an experimental (manipulation) and control (placebo) groups of fifteen subjects respectively. The manipulation group received the manipulation (high velocity, low amplitude), which was performed by a physical therapist with the subject in the supine position and with the arms crossed over the chest and hands passed over the shoulders. For the sham group, the same procedure was performed, with the exception that the high-velocity thrust was not applied. Measurements were taken before and after the intervention. Muscle activity of upper and lower trapezius, serratus anterior, middle deltoid was measured using surface electromyography. Visual analog scale (VAS) was used for shoulder pain. Goniometry was used for shoulder abduction active range of motion (AROM). Straight edge was used for supine rounded shoulder posture (RSP) distance. Results: The muscle activity of the upper trapezius, lower trapezius and middle deltoid muscle increased significantly after the intervention (p<.05). However, no significant difference was observed in serratus anterior muscle (p>.05). The VAS was significantly decreased and AROM significantly increased after the intervention (p<.05). The distance of RSP were not significant (p>.05). The control group showed no differences before and after the intervention (p>.05). Conclusion: The results of this study suggest that thoracic spine thrust manipulation can be an effective component of treatment plan to improve pain and function.