Rick Perry, after a disappointingly low finish, said he would be concentrating on the South Carolina primary. Michele Bachmann, who came in last among the active campaigners, Wednesday ended her quest, becoming the first casualty after the official start of the primary season.
The final vote total for Romney was 30,015 to Santorum's 30,007, out of 122,255 votes cast overall, Iowa Republican Party Chairman Matt Strawn announced.
Romney, a former Massachusetts governor, and Santorum, a former U.S. senator from Pennsylvania, each had 24.5 percent of the total, with libertarian-learning U.S. Rep. Ron Paul of Texas close behind with 21.4 percent.
For much of the night Tuesday, Romney and Santorum were separated by only a sliver of votes.
Romney finished with the lowest percentage total of any GOP winner in Iowa's modern history, sinking below Bob Dole's 26 percent in 1996.
Further back in the pack were former House Speaker Newt Gingrich with 13 percent of the vote and Texas Gov. Perry with 10 percent. Bachmann, a Minnesota congresswoman, had 5 percent.
Romney told supporters in Des Moines early Wednesday, before the final vote was counted, that the results were a "great victory" for him, Santorum and Paul.
"All three of us will be campaigning very hard to restore the heart and soul of this nation," Romney said.
Santorum told reporters: "People have asked me, why do you think you can win? Let me tell you, what wins in America are bold ideas, sharp contrasts, and a plan that includes everyone."
He vowed to make a strong push in New Hampshire. "With your help and God's grace, we'll have another fun night a week from now."
Paul praised his supporters in an upbeat speech for a "fantastic showing." He said he, Romney and Santorum were the caucus' "three winners," ready to raise money and compete well in the next contests.
Gingrich pledged to press forward and be on the stage at the next debate in New Hampshire Saturday.
"There will be a great debate in the Republican Party before we are prepared to have a great debate with Barack Obama," Gingrich said, pledging to raise the intensity of his criticism of Romney before the next contests.
Before saying he would focus on South Carolina, Perry told supporters he would return to Texas to "determine whether there is a path forward" for his campaign.
In less than five months, his candidacy went from first to almost last, brought down by several unsatisfactory debate performances.
Perry had said he would take the campaign directly to South Carolina, skipping New Hampshire to fight on in the conservative Southern state.
Bachmann said she believed she was "that true conservative who can and will defeat Barack Obama." But by Wednesday morning she decided to end her campaign, saying the people of Iowa have spoken.
"I have decided to stand aside," she told a West Des Moines news conference.
The New Hampshire primary is Jan. 10. The South Carolina primary, considered a "firewall" to protect GOP front-runners in the presidential nomination race, is set for Jan. 21.
CNN reported Sen. John McCain, R-Ariz., the 2008 Republican presidential nominee, planned to endorse Romney in New Hampshire Wednesday.
Caucus-night polling suggested Romney was the strongest performer among voters whose top priority is beating Obama.
Caucus-goers who described themselves as "very conservative" voted for Santorum, polling by Edison Research for the National Election Pool of news organizations indicated.
Santorum picked up 30 percent of strong Tea Party backers, the polling indicated.
Paul far outpaced his rivals among caucus-goers under age 40. But he dropped behind Romney and Santorum among voters 40 and older.
Nearly six in 10 voters said they considered themselves evangelical or born-again Christians, the polling indicated.