The former governor and ex-presidential candidate has announced his support for his former rival from Texas. The move comes as GOP officials seek to block Donald Trump's rise to the Republican nomination.
Jeb Bush's announcement came on Wednesday, shortly after Trump claimed yet another primary win in the state of Arizona.
"For the sake of our party and country, we must move to overcome the divisiveness and vulgarity Donald Trump has brought into the political arena, or we will certainly lose our chance to defeat the Democratic nominee and reverse President Obama's failed policies," Bush said in a statement.
He also took to Twitter to announce his endorsement.
Bush's endorsement of Texas senator Ted Cruz comes as the Republican establishment grapples with an unexpected rise in populist anger directed at Washington, the flames of which Trump has helped fan with his vitriolic rhetoric. Bush, once seen as the natural frontrunner thanks to his family's strong ties to the US political establishment - he's the brother of George W. Bush and the son of George H.W. Bush - was a frequent target of Trump, who often referred to the Florida governor as "low-energy."
Bush dropped out of the race following a disappointing defeat in South Carolina's primary last month.
Cruz hailed the endorsement, calling it further proof that the Republican party's only path to stopping Trump lay with him. A third candidate, Ohio Governor John Kasich, has insisted he will stay in the race.