"It's over for me," Santorum, a former U.S. senator from Pennsylvania, declared at a news conference in Gettysburg, Pa., two weeks ahead of that state's primary.
Santorum also stopped short short of endorsing Romney although he vowed to help defeat President Obama in November. The move saves Santorum from the embarrassment of a possible loss to the former Massachusetts governor in the April 24 balloting in his home state.
"We made a decision at our kitchen table to get into this race against all odds. We made a decision at our kitchen table this weekend to suspend this campaign," said Santorum, whose 3-year-old daughter Bella, had been hospitalized for several days before she was released Monday. Bella suffers from the rare genetic disorder Trisomy 18, which is generally fatal before or during birth.
Santorum said he and his family spent a "difficult weekend" but Bella "is a fighter."
"She is doing exceptionally well," he said. "She is back with us and we are looking forward to spending time with her."
Santorum said he spent the weekend in "prayer and thought" in coming to his decision.
"Miracle after miracle, this race was as improbable as any you will ever see for president," he said.
"While this presidential race is over for me, and we will suspend our campaign today, we are not done fighting."
Earlier in the day, Santorum had canceled several campaign appearances.
Santorum reflected on the last year, saying though pundits kept saying he wasn't winning the race, he was "winning in a different way -- touching hearts, raising issues."
"Against all odds we won 11 states. …," he said. "We won more counties than all [the] others in this race combined. I found a deeper love for this country."
Romney issued a statement praising Santorum and congratulating him on his campaign run.
"He has proven himself to be an important voice in our party and in the nation," Romney said. "We both recognize that what is most important is putting the failures of the last three years behind us and setting America back on the path to prosperity."