There is a lack of evidence to inform the use of spinal manipulative therapy (SMT) for patients with shoulder pain. A validated sham comparator is necessary to ascertain the unique effects of SMT. This study investigates the plausibility of a thoracic sham-SMT comparator for SMT in patients with shoulder pain. Participants (n=56) with subacromial impingement syndrome were randomized to thoracic SMT or a sham-SMT. An examiner blinded to group assignment took measures before and after treatment of shoulder active range of motion (AROM) and perceived effects of the assigned intervention. Treatment consisted of six upper, middle and lower thoracic SMT or sham-SMT. The sham-SMT was identical to the SMT, except no thrust was applied. Believability as an active treatment was measured post-treatment. Believability as an active treatment was not different between groups (χ(2)=2.19; p=0.15). Perceptions of effects were not different between groups at pre-treatment (t=0.12; p=0.90) or post-treatment (t=0.40; p=0.69), and showed equivalency with 95% confidence between groups at pre- and post-treatment. There was no significant change in shoulder flexion in either group over time, or in the sham-SMT for internal rotation (p>0.05). The SMT group had an increase of 6.49° in internal rotation over time (p=0.04). The thoracic sham-SMT of this study is a plausible comparator for SMT in patients with shoulder pain. The sham-SMT was believable as an active treatment, perceived as having equal beneficial effects both when verbally described and after familiarization with the treatment, and has an inert effect on shoulder AROM. Use of this comparator can be considered in clinical trials investigating thoracic SMT.