This is the first case report of Mondor's disease diagnosed in a marathon runner developed due to tight heart rate strap. A 41-year female marathon runner was admitted to the hospital with a cord-like soft tissue swelling under her breast, which was confirmed by physical examination. She had a history of severe exercise; during which, she used a sports watch with a heart rate strap that she tightened under her bra. Doppler ultrasonography revealed a superficial non-compressible thrombosed vein compatible with Mondor's disease. Key Words: Mondor's disease, Ultrasonography, Heart rate strap.