A heroic mystery man has jumped into the Yarra River to save a young woman.

The 21-year-old dived into Melbourne's notoriously dirty waterway on Tuesday night after an 18-year-old woman was seen toppling into the river from the King Street Bridge in Southbank.

The woman fell into the Yarra River at the King Street Bridge and is recovering in hospital. Credit:James Davies

After witnessing the woman's plunge, the quick-thinking man threw himself in to rescue her.

The woman was taken to St Vincent's Hospital for assessment while the man was uninjured.