The woman saw the 31-year-old man lose control over his scooter on a curve in the Ruschorter Hauptdeich area of Hamburg and crash into a metal sign warning of a bend.
The sign was so sharp at the edge and the man hit it at such an unfortunate angle that his lower right leg was nearly severed.
When the 39-year-old woman, named by the Hamburger Morgenpost only as Manuela S., reached him, he was bleeding heavily.
She immediately took off her bra and used it to tie up the leg and stop the bleeding.
He is now in hospital, where doctors report he will make a full recovery.