Background: Pectoralis minor syndrome (PMS) develops when the neurovascular bundle compression occurs at the retropectoralis minor space. It may occur due to repetitive overhead activities, traumatic incident, structural causes, myofascial pain syndrome in the pectoralis minor muscle, as well as spasticity of the pectoralis minor muscle. In patients with hemiplegia, adductor muscles along with pectoralis minor muscle spasticity may be present in the upper extremity.Objective: We report a 19-year-old male patient with spastic hemiparesis who was diagnosed with PMS due to spasticity of the pectoralis minor muscle.Method: Diagnosis of PMS was confirmed by Ultrasound-guided 4 cc 1% lidocaine injection to the right pectoralis minor muscle and Ultrasound-guided onabotulinum toxin A injection was performed. Stretching exercises to the pectoral muscles were also added to the rehabilitation program.Result: Complaints of the patient were controlled by botulinum toxin injections at 3-month intervals.Conclusion: It should be kept in mind that spasticity in the upper extremity may develop in the pectoralis minor muscle, and may cause pressure on the neurovascular structures. Ultrasound-guided botulinum toxin injections can be a safe and effective treatment for PMS in a patent with post stroke spastic hemiparesis.