"I wanted you all to be the first to know that today I am endorsing Ted Cruz for President," Bush said in a statement.
"Ted is a consistent, principled conservative who has demonstrated the ability to appeal to voters and win primary contests, including yesterday's Utah caucus," Bush said, referring to one of three votes held yesterday.
Bush, who dropped out of the presidential race last month after a disappointing campaign, said the Republican Party has to unite or face certain defeat in the election, probably to Hillary Clinton.
"For the sake of our party and country, we must 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, most likely Hillary Clinton, this fall," the statement said.