WASHINGTON — The top federal ethics watchdog said on Thursday that Treasury Secretary Steven Mnuchin’s sale of his stake in a film production business to his wife did not comply with federal ethics rules, and it would not certify his 2018 financial disclosure report as a result.

Although Mr. Mnuchin will not face penalties for failing to comply, he has been required to rewrite his federal ethics agreement and to promise to recuse himself from government matters that could affect his wife’s business.

Mr. Mnuchin in 2017 sold his stake in StormChaser Partners to his then-fiancée, Louise Linton, as part of a series of divestments before becoming Treasury secretary. Since they are now married, government ethics rules consider the asset to be owned by Mr. Mnuchin, potentially creating a conflict of interest for an official who has been negotiating for expanded access for the movie industry as part of trade talks with China.

The controversy over Mr. Mnuchin’s finances has become an unwanted distraction in recent weeks as the Trump administration has been engaged in intense negotiations with China on a wide range of trade matters. While Robert Lighthizer, President Trump’s top trade official, has been leading the talks, Mr. Mnuchin has been the point person for promoting the film industry because of his background as a Hollywood producer and investor.