The 14-year-old boy was cycling along Montague Street, Worthing, around 8.10pm on Monday 27 August when he was approached by a man, police said.
The man stopped the bike, threatened the teenager and demanded he handed over the bike, according to police.
Catherine Maloney said: “The teenager was on holiday in the area and has understandably, been left very upset by this incident.
“Have you seen a black bike with stickers on the frame and blue spokes for sale in the area recently?
“The suspect is described as a white man, 5ft 8ins, of medium build, with black greying hair and wearing a bright blue parka jacket.
“We are very keen to speak to anyone who has seen this bike, recognises the description or witnessed the incident.”
To report information please go online or call 101 quoting 1169 of 27/08. Alternatively, you can visit the Crimestoppers website or contact the independent charity anonymously on 0800 555 111.