During his own push for the Republican nomination Santorum, a staunchly Roman Catholic former senator who withdrew from the presidential race last month, said Romney was not conservative enough.
Now however, he has told his supporters to back Romney to beat Democratic President Barack Obama in November.
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Romney and Santorum met on Friday to do some fence mending after a bitter campaign, but Romney said then he was not expecting his former rival's formal backing yet.
"I felt that it was completely impossible for me to even consider an endorsement until after a meeting to discuss issues critical to those of us who often feel our voices are not heard by the establishment: social conservatives, tea-party supporters, lower and middle income working families," said Santorum.
Romney "clearly understands that having pro-family initiatives are not only the morally and economically right thing to do, but that the family is the basic building block of our society and must be preserved," Santorum stressed.
"We talked about what it would take to win this election. ...The America we know is being fundamentally changed to look more like a European socialist state than the land of opportunity our founding fathers established," he argued.
At campaign events this year, arch-conservative Santorum repeatedly spoke about social themes such as abortion and gay rights, two bedrock conservative issues on which Romney has changed his position over the years.
When Santorum dropped out of the race on April 10 after a surprisingly strong grass roots campaign that began last year in a pick-up truck, he did not mention Romney's name.
But the Republicans must close ranks if they want to keep Obama from winning a re-election in November.
Despite differences with Romney "there are many significant areas in which we agree: the need for lower taxes, smaller government, and a reduction in out-of-control spending," Santorum wrote.
"We certainly agree that abortion is wrong and marriage should be between one man and one woman. I am also comfortable with Governor Romney on foreign policy matters, and we share the belief that we can never allow Iran to possess nuclear weapons.
"Above all else, we both agree that President Obama must be defeated. The task will not be easy. It will require all hands on deck if our nominee is to be victorious," he wrote, stressing: "Governor Romney will be that nominee and he has my endorsement and support to win the most critical election of our lifetime."
Newt Gingrich, a diehard challenger for the Republican nomination, officially quit the campaign last week and endorsed Romney.
Libertarian-leaning congressman Ron Paul remains in the race, but he has no chance of defeating Romney.
