MPAA officers had not returned calls at time of writing, however the ruling is believed to be in response to the highly erotic nature of Lee's follow-up to 2005's Brokeback Mountain.
The MPAA screened the film, a tale of espionage set in Shanghai during the second world war, on Aug 22 and told Focus about its decision later that day.
While the NC-17 rating can spell trouble for a distributor - some exhibitors refuse to handle films with the rating - Focus top brass do not want to compromise Lee's artistic vision and believe fans will seek it out.
'As with so many of his previous films, Oscar-winning director Ang Lee has crafted a masterpiece about and for grown-ups,' Focus chief executive officer James Schamus said.
'Focus Features accepts the MPAA's NC-17 rating without protest and Lust, Caution will be released this fall as previously announced.'
The film will receive its world premiere in Venice and its North American premiere in Toronto. The domestic release date remains Sept 28.
Lust, Caution is a Focus Features and River Road Entertainment presentation in association with Haishang Films, and is based on the short Se, Jei by Eileen Chang.