The man tried to climb down a drain pipe next to a city building on Clarence Place, near Rundle Mall, around 2 p.m.
He climbed 12 meters before the pipe broke, causing him to plunge more than two stories below the road.
Paramedics, police and firefighters rushed to the scene while their friends watched in shock.
The young man from the inner suburbs of Adelaide remains in Royal Adelaide Hospital with serious injuries.