Facing a union disciplinary hearing over his role in the attack on the Capitol, former President Donald J. Trump resigned from the Screen Actors Guild-American Federation of Television and Radio Artists on Thursday, assailing the group in a grievance-filled letter.

“I write to you today regarding the so-called Disciplinary Committee hearing aimed at revoking my union membership,” Mr. Trump wrote in the letter to the union. “Who cares!” He went on to say that he was resigning immediately.

Mr. Trump’s resignation was first reported by Fox News.

Mr. Trump — a businessman who, before entering politics, had made several appearances in movies and television shows, most notably on “The Apprentice” — had been charged by the union with “inciting the attack on the U.S. Capitol” on Jan. 6. and of “sustaining a reckless campaign of misinformation aimed at discrediting and ultimately threatening the safety of journalists, many of whom are SAG-AFTRA members,” according to a statement released last month by the union.

As a result, the union’s board voted Jan. 19 to find probable cause that Mr. Trump had violated SAG-AFTRA’s Constitution, and it ordered that the matter be heard by the union’s disciplinary committee. Had Mr. Trump been found guilty of the charges by the committee, he would have faced penalties ranging from censure to expulsion.

Fox News reported and SAG-AFTRA confirmed that Mr. Trump’s disciplinary hearing had been scheduled to take place this week.

“Donald Trump attacked the values that this union holds most sacred — democracy, truth, respect for our fellow Americans of all races and faiths, and the sanctity of the free press,” Gabrielle Carteris, the union’s president said in last month’s statement. “There’s a straight line from his wanton disregard for the truth to the attacks on journalists perpetrated by his followers.”

Mr. Trump’s letter, which was obtained by The New York Times and later posted by the union, was dated Feb. 4 and addressed to Ms. Carteris. In it, he blasted the organization, asserting that it had “done little for its members, and nothing for me.”

“I no longer wish to be associated with your union,” Mr. Trump wrote. “As such, this letter is to inform you of my immediate resigning from SAG-AFTRA.”

Mr. Trump also used the letter to enumerate his own acting accomplishments and argue that he had helped the cable news business thrive. He wrote that he was “very proud of my work on movies such as ‘Home Alone 2,’ ‘Zoolander’ and ‘Wall Street: Money Never Sleeps’; and television shows including ‘The Fresh Prince of Bel-Air,’ ‘Saturday Night Live,’ and of course, one of the most successful shows in television history, ‘The Apprentice’ — to name just a few.’’

Responding to Mr. Trump’s resignation, SAG-AFTRA offered a simple two-word statement jointly attributed to Ms. Carteris and David White, the union’s national executive director on Thursday afternoon: “Thank you.”

Mr. Trump’s departure from the union will not necessarily preclude him from working in film or television, but could give employers reason to think twice before casting him.

SAG-AFTRA represents approximately 160,000 actors, singers, journalists and other media professionals in film and television. In its January statement announcing the disciplinary hearing for Mr. Trump, the union noted that reports of intimidation and physical assaults against reporters escalated during the Trump presidency.