The man got on the number 87 bus in the town’s station at the same time as the woman, at around 7.40pm on 31 May.
He allowed her to get on in front of him before choosing to sit down next to her, despite there being several free seats downstairs.
The man got close to the young woman before leaning across and touching her leg.
She shouted out at him but he refused to move, forcing her to push past him to report the incident to the driver.
The woman got off the bus before making her way to a police station to report the crime.
Detective sergeant Emma Holder, from the Project Empower team at Safer Travel, said: “It would appear that this young woman was targeted, as there were other seats available on the bus.
“We want to trace the man captured on CCTV so that he can be spoken with regarding the allegation.
“Anyone who recognises the man is urged to come forward with his name or encourage him to present himself to police.”
The offender is described as an Asian man, 5ft 7in to 5ft 8in tall, in his late 30s, with short messy black hair. CCTV shows him wearing a black tracksuit top with white stripes, a white cap and carrying a black bag.
Project Empower sees officers from West Midlands Police’s Public Protection Unit (PPU) working as part of the Safer Travel Partnership to fast-track enquires into complaints like sexual touching, lewd comments and indecent exposure.
Working from the Birmingham HQ of transport authority Centro – in Summer Lane – they have access to a network of CCTV cameras covering the bus and rail network to help them swiftly identify offenders.
Empower also sees public transport staff – including drivers and conductors – trained to spot the tactics used by sex offenders, support passengers who’ve been targeted and report suspicious behaviour.