Pectoralis major (PM) injuries are uncommon, typically affecting young male athletes engaging in high-intensity activities like weight-lifting. A 62-year-old male, who previously suffered a stroke leading to left hemiparesis, hemisensory loss, and spasticity, exhibited a left chest swelling during a rehabilitation clinic visit. Subsequent inquiries revealed his recent incorporation of a home-based pulley system for stretching exercises. On examination, the swelling was diffuse, firm, and non-tender, located at the midclavicular line of his left chest, with a positive dropped nipple sign and loss of the left anterior axillary fold sign. An ultrasound confirmed a low-grade injury to the left PM tendon. Spastic muscle ruptures are extremely rare, with only three published reports linked to traumatic brain injury, multiple sclerosis, and spinal cord injury. Spasticity increases muscle vulnerability due to structural and mechanical changes to the skeletal muscles. This is the first report of a spastic PM tendon rupture and the first following a stroke. This case highlights the need for optimized multimodal spasticity management and reinforces the importance of comprehensive patient education on the safe execution of home-based stretching exercises.