It was on the way up to Alpe d'Huez in the 19th stage of the Tour that the crash occurred.
An escort car suddenly braked and Einer Rubio cycled at high speed into the car's rear window, which was shattered in the collision.
Rubio reportedly suffered cuts to his face and was taken to hospital by ambulance. His team, Movistar, later announced that the Colombian was forced to withdraw from the Tour.
"It didn't look good at all. I wonder if he broke the window with his face. I hope he's okay," said Norwegian skier Tobias Halland Johannessen, who saw the crash.
The stage was won by Slovenian Tadej Pogacar, who also leads the Tour overall. The Tour will conclude on Sunday with the finish in Paris.
