The victim was approached near SPAR in the town’s Charlotte Dundas Court at about 10am on Friday, March 26.
Police say one of the group, a teenage boy, attacked the woman when she declined their request.
A Forth Valley Police Division spokesperson said: “We are appealing for witnesses to an assault in Grangemouth.
“A female was approached by a group of teenagers who asked her to buy them alcohol. On refusing, the female was assaulted by an unknown male youth.
“Two unknown members of the public stopped to check on the welfare of the female who was assaulted and thereafter left without providing details.
“If you are, or know, these two members of the public or have any information in relation to this assault, call 101 quoting incident number 0981 of March 26, 2021.”