The 81-year-old man was hit by the tram near to the Woodbourn Road tram stop at around 10.10am this morning (Thursday).
South Yorkshire Police said: "He was immediately taken to hospital where his injuries are described as life-threatening.
"Officers are appealing to anyone who may have been in the area at the time or who witnessed the collision to please come forward."
The tram was travelling along Woodbourn Road in the direction towards Meadowhall at the time of the collision.
Anyone with information is asked to call 101, quoting incident number 299 of 22 December 2016.