Police are urging the woman to come forward to help with the investigation.

The 24-year-old had just knocked off work at a nearby eatery on the Yarra promenade in the city just before midnight on Friday when he saw a woman being assaulted by a man.

As he stepped in to help the woman, the offender produced a knife and stabbed him in the left side.

Police said the offender had earlier tried to pick a fight with people walking along promenade, and have urged two other men they believe were assaulted prior to the stabbing to contact them.

The victim was left with non-life threatening injuries and sought help from a nearby late-night parking attendant, police said.

The offender fled, and is described as being approximately 180cm tall, wearing a black and white cap, a grey long-sleeve top with a large navy blue, red and white Tommy Hilfiger motif, dark blue shorts and white runners.

The offender was also carrying a black satchel, worn across his chest.

Police requested anyone with information to contact Crime Stoppers on 1800 333 000 or submit a crime report at www.crimestoppersvic.com.au