Police are hunting a man who allegedly punched a passenger in the head on a Melbourne train after he was asked to turn his music down.

The victim, a 19-year-old man, boarded a city bound Frankston line train at Patterson Lakes at 2.30pm on August 30.

Police have released footage of a man they want to speak to after a passenger was allegedly struck. Credit:Victoria Police

Another man boarded the train at Bentleigh playing loud music on a Bluetooth speaker he was holding in his hand.

When the victim asked the man to turn down his music, police allege, he was punched in the back of the head.