Medics and firefighters responded to the scene, at 100 S. King Street, at about 11 a.m. after receiving a report of a serious accident.
A high-angle rescue crew was needed to bring the two construction workers down to ground level so they could be treated. They were then taken to Harborview Medical Center and were expected to survive.
Seattle Fire Department spokesman Kyle Moore said the two men apparently were struck by re-bar that was being hoisted by crane to the fourth floor of the building.
Video shot by a witness showed pieces of re-bar falling from the crane onto the top of the construction site.
Moore said one of the workers, a 34-year-old man, suffered severe head trauma. The other worker suffered only minor injuries.
Watch raw cell phone video of the re-bar falling from the crane: