Mr Trump earned his membership from appearances as himself in films such as Home Alone 2 and TV programmes such as Sex And The City in the early 1990s.
His most recent roles were as host and producer of reality series The Apprentice and its spinoff Celebrity Apprentice.
The 74-year-old said he was “very proud” of his acting work but criticised the union for doing “little for its members and nothing for me – besides collecting dues and promoting dangerous un-American policies and ideas”.
He concluded: “I no longer wish to be associated with your union. As such, this letter is to inform you of my immediate resignation from SAG-AFTRA. You have done nothing for me.”
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The guild published its two-word response online: “Thank you.”
SAG-AFTRA said last month that Mr Trump had “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”.