"It is time to get behind the nominee," Bush told the Tribune-Review on Wednesday during a visit to Pittsburgh. "And it is time for the country to focus on the most important thing, which is to elect a president who will create a climate for high growth and job-creating abilities for our country."
Bush called for the Republican Party to unite behind the former Massachusetts governor, who won the Illinois primary this week - with 99 percent of precincts reporting, Romney led rival Rick Santorum 46.7 to 35 percent.
GOP strategist Bruce Haynes said the Bush endorsement is worth a lot to the Romney campaign.
"It's literally the sound of the referee's whistle calling the game to an end," Haynes said.
Santorum, former House Speaker Newt Gingrich and Rep. Ron Paul of Texas have declared that they will remain in the race until the Republican National Convention in August in Tampa.
Bush, 59, the son of a president and brother of another, pushed aside any interest in running with Romney. But he has strong feelings on whom he wants Romney to pick as a running mate.
"Marco Rubio," he said of the freshman Florida GOP senator, who served as a volunteer on Bush's governor's campaign. Bush described Rubio, 40, as "dynamic, joyful, disciplined and principled."
"He is the best orator of American politics today, a good family man. He is not only a consistent conservative, but he has managed to find a way to communicate a conservative message full of hope and optimism," Bush said.
Bush had remained silent on whom he would support for the Republican presidential nomination, even during the Florida primary. Romney won the state and all of its delegates in January.
Bush said he is confident Romney would advocate for education issues, an issue the former governor said he was "passionate" about.
"He knows the proper role of government in education, which is limited," Bush said. "You do not have to have an interventionist federal policy to make something as important as education a national priority."
Bush, who has urged the GOP to be more sensitive to Latino voters, said the party has a long way to go.
Latino voters are expected to represent the margin of victory in at least 10 to 15 key battleground states in November. Stopping the flow of illegal immigration from Mexico and South America emerged as a significant issue in GOP presidential debates.
"The problem lies in the tone," Bush said. "You do not have to sacrifice principle to win the Latino vote."