A woman was sexually assaulted by a man who sat next to her on a bus that was replacing trains from Melbourne's CBD to Caulfield two months ago.

Image of the man police want to speak to. Credit:Victoria Police

The woman and the alleged offender boarded a replacement bus on Flinders Street near Federation Square, towards Caulfield, on the afternoon of Tuesday, April 16.

The man sat next to the woman on the bus and assaulted her during the trip, according to police, before they got off at Caulfield Railway Station about 6.25pm.

Police have released a computer-generated image of the man, describing him as being of Indian appearance, 175 centimetres tall, and having a thin build and black hair.

He was wearing rectangular glasses with a black thick frame, black pants, a black or dark grey knitted cardigan and a black backpack.

Investigators are keen to speak to anyone who may have witnessed the incident or was travelling on the bus at the time.

They urge anyone with any information to contact Crime Stoppers on 1800 333 000 or submit a confidential report online at www.crimestoppersvic.com.au.