Police are appealing for assistance after a teenager was indecently assaulted on a tram in Northcote, north of Melbourne, on Monday.

The 14-year-old was on the city-bound 86 tram at around 8.30am when an unknown man indecently assaulted her.

The girl screamed, causing the man to quickly disembark from the tram. He was last seen walking back towards Preston along High Street.

Police described the daylight attack on the busy tram as "quite brazen".

"It's not uncommon for victims in these situations to freeze, feel shocked and not know what to do, so it's really great she stood up to him," Detective Senior Constable Robert Tammaro said.

Security cameras at surrounding businesses caught the man accused of the attack. (9NEWS)

The accused attacker had a distinctive silver ball piercing. (9NEWS)

Anyone who recognises the man is urged to call Crime Stoppers. (9NEWS)

The man was caught on security cameras as he fled the scene. He is described as Caucasian and around 175cm tall, aged in his mid-30s, with a thin build and dark blonde/brown hair parted down the middle, and acne scarring on his face.

At the time of the attack he had a distinctive silver ball piercing underneath the left side of his lip and smelled of nicotine.