The advert for "Coco Before Chanel," which shows the film's star Audrey Tautou posing in silk pyjamas with Chanel's trademark cigarette dangling from her fingers, has appeared across the city.
But James Middleton, chairman of the anti-tobacco committee for campaign group Clear the Air, said the poster breaks Hong Kong laws banning any mention of smoking or cigarettes in advertising.
"It is a direct breach of the law," he told AFP.
Earlier this year, the company that runs advertising on trains and buses in Paris pulled a similar poster over fears it could be prosecuted.
An advert for the film that appeared Friday in the South China Morning Post had the cigarette airbrushed out.
No one from the company responsible for promoting the movie in Hong Kong, Lark Films Distribution, was immediately available for comment.
Hong Kong's Health Department was also not immediately available to answer questions from AFP, but a spokesman told the South China Morning Post it was looking into the case.