On June 15 at around 8.50pm, a 15-year-old girl and a 16-year-old boy were on the 50B bus service heading towards Woodlands when an unknown man boarded.
The man is believed to have behaved aggressively towards the teenagers, before touching the 15-year-old girl inappropriately and grabbing the 16-year-old by the neck.
The suspect is then believed to have shown the teenagers a sheath of a knife hidden underneath his clothing.
He is described as white, aged between 20 and 30, wearing work-type clothing and is reported to have spoken in a foreign language.
Officers believe the individual pictured could hold vital information and are urging him, or anyone who recognises him, to get in touch.
Anyone with information please call 101, quoting 14/110180/22, visit www.southyorks.police.uk/contact-us/report-something/ or Crimestoppers on www.crimestoppers-uk.org or 0800 555 111.