Santorum announced the endorsement in an e-mail to supporters, Politico reported. Santorum and Romney met Friday in Pittsburgh but aides at the time would not disclose details of the meeting.
In the e-mail Monday, Santorum -- who left the race for the Republican nomination April 10 -- told supporters the issue of his endorsement did not come up in Friday's meeting, which he said "was candid, collegial and focused on the issues that you helped me give voice to during our campaign; because I believe they are essential ingredients to not only winning this fall, but turning our country around."
Santorum said Monday "it was completely impossible for me to even consider an endorsement" before discussing with Romney "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."
"I can assure you that even though I am no longer a candidate for president, I will still continue to fight every day for our shared values -- the values that made America the greatest country in the history of the world," Santorum wrote.
The former U.S. senator from Pennsylvania told supporters he "strongly urged" Romney to add conservatives to his campaign staff.
"The primary campaign certainly made it clear that Governor Romney and I have some differences," Santorum wrote. "But there are many significant areas in which we agree: the need for lower taxes, smaller government, and a reduction in out-of-control spending. We certainly agree that abortion is wrong and marriage should be between one man and one woman."
He said he is "comfortable" with Romney on foreign policy" and has "no doubt if elected he will work with a Republican Congress to repeal" the Affordable Care Act and "replace it with a bottom up, patient, not government, driven system."
"Above all else, we both agree that President Obama must be defeated."
Santorum closed with a promise to make "another big announcement" soon.