Details of the 90-minute meeting, however, weren't revealed, CNN reported.
Aides said only Romney and Santorum met and weren't available for photos or statements afterward.
Santorum bowed out of the race April 10.
John Brabender, a senior strategist to the Santorum campaign, told CNN before the meeting Santorum was seeking a "comfort level" about the role "social conservatives, Tea Party activists and blue-collar Republicans will play in the campaign and in the Romney administration."
"We are not walking in there with a litmus test," Brabender said. "This is meant to be a candid conversation."
Aides to Santorum, a two-term U.S. senator from Pennsylvania, said he wanted to hear from Romney's campaign about whether conservative principles would be emphasized in the general election campaign and reflected in the party's platform.
Romney told the Pittsburgh Post-Gazette before the meeting he didn't expect Santorum would endorse him at the meeting, but added, "you're going to see us all come together."
After the meeting, Romney criticized U.S. President Barack Obama's handling of the economy, particularly in job creation.
"So many became discouraged, they stopped looking for work," Romney said during a campaign event, referring government statistics released Friday indicating the workforce fell 340,000 in April. "This is a sad time in America when people who want work can't find jobs."