He will play Trump for the entire season, sources have told The Hollywood Reporter.
The reality star turned Republican presidential nominee was played last season by Darrell Hammond, a onetime castmember, who returned to the show in 2014 as its announcer.
Hammond will remain with the series as its announcer and continue to appear on the show periodically. Before Hammond, Trump was played by castmember Taran Killam, whose contract was not picked up for season 42.
Trump himself hosted the show in November 2015, amid much controversy.
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A close personal friend of Michaels', who also starred on the Michaels-produced NBC sitcom "30 Rock", Baldwin officially signed on earlier this week and taped a promo that debuted on social media yesterday.
The arrangement is similar to the casting last season of Larry David to play Democratic presidential candidate Bernie Sanders over multiple episodes, an idea that paid off in huge comic dividends with winning sketches like "Bern Your Enthusiasm," a send-up of David's long-running HBO hit.
Baldwin has deep ties to SNL, having guest-hosted a record-setting 16 times since his debut in 1990.