The 125,000-member union, whose New York locals represent Broadway's stagehands, ushers, treasurers, ticket sellers, wardrobe artists, scenic artists and other backstage workers, pledged its support to the former First Lady, New York Senator and U.S. Secretary of State.
The union's general executive board voted unanimously for the Democrat at a Jan. 11 meeting.
IATSE President Matthew Loeb released a statement saying, "Hillary Clinton is the only candidate that reached out to discuss issues affecting members of the IATSE.... She spent her valuable time discussing our unique issues and challenges with me, demonstrating not just her support of workers in general, but also that she is concerned with the well-being of members of the IATSE."
IATSE endorsed President Barack Obama in his two presidential campaigns, in 2008 and 2012.
Clinton is facing a primary challenge from Vermont Sen. Bernie Sanders to be the Democratic Party standard bearer in the November presidential election. She has also been a frequent theatregoer, most recently at Hamilton with her husband, former President Bill Clinton.