Romney spokeswoman Gail Gitcho says the two will appear together and speak to reporters.
Formal backing from the 41st president is another sign that the Republican Party is uniting behind Romney as pressure builds on challengers Rick Santorum and Newt Gingrich to leave the race.
The elder Bush has offered encouraging words to Romney throughout the primary season but had withheld a formal endorsement. Former first lady Barbara Bush has formally backed Romney. She recorded automated telephone calls for him during the primary in Ohio.
Their son, former Florida Gov. Jeb Bush, endorsed Romney last week.