Mr Romney has won 47% of the votes, compared with 35% for Rick Santorum, with nearly all votes counted.
Sending a message to President Obama at a victory party in a suburb of Chicago, Mr Romney said: "We've had enough."
Ron Paul polled 9% in Illinois and Newt Gingrich was on 8%; neither candidate campaigned extensively in the state.
"I'm running for president because I have the experience and the vision to get us out of this mess," said Mr Romney, as his victory became evident.
Illinois sends 54 delegates to the Republican convention, and Mr Romney hopes to maintain his momentum.
However, Illinois's delegates are not determined by the statewide vote.
Individual delegates are listed on the ballot in each of the state's 18 congressional districts and are identified by the candidate they support.
Mr Santorum's campaign did not successfully file for the primary ballot in parts of Illinois, meaning he automatically cedes of those 10 delegates.
However, addressing supporters on Tuesday evening, Mr Santorum said he had polled well in Illinois in areas "that conservatives and Republicans populate".
"We're very happy about that and we're happy about the delegates we're going to get, too," he said.
The candidates had clashed in Illinois over the economy.
On Monday Mr Santorum had said he "didn't care about the unemployment rate", and told supporters the presidential campaign was about smaller government and winning back individual and social freedom.
Mr Romney jumped on those remarks later in the day, telling students in Peoria, Illinois "one of the people who is running also for the Republican nomination today said that he doesn't care about the unemployment rate".
"It does bother me. I want to get people back to work," Mr Romney said.
Later Mr Santorum told supporters: "The economy is a big issue. Unemployment is a big issue."
'Magic number'
Mr Romney won a convincing victory in Puerto Rico's primary over the weekend, amassing 83% of the votes, but lost to Mr Santorum in recent contests in the South.
A candidate needs to accumulate 1,144 delegates to the August convention in order to secure the nomination.
Analysts say the current figures make that an almost impossible task for Mr Santorum, who has spoken openly in recent weeks about winning enough delegates to stop Mr Romney taking the crown.
Such an outcome would lead to a competitive vote at the Tampa convention, in which Mr Santorum feels he could overcome Mr Romney.
After the Puerto Rico result, Mr Romney has 521 delegates. Mr Santorum has 253 delegates, former House Speaker Newt Gingrich has 136 and Ron Paul had 50, according to an Associated Press tally.
Mr Romney's well-financed campaign and its allies have already spent $2.5m in adverts in the state, and insists that Mr Santorum cannot hope to win the delegate race and should drop out.
Mr Santorum has vowed to continue, "competing in every state", citing tepid support for Mr Romney even in states the former Massachusetts governor has won.
On Monday, Mr Santorum, a former Pennsylvania senator, said he would "go out and compete in every state".
"I think it's going to be very difficult as this goes on for anybody to get that magic number," Mr Santorum said in an interview with CBS News, adding that chances were increasing of the nomination being decided at the convention.
After his Puerto Rico win, Mr Romney described the result as an "extraordinary victory".
"Those people who don't think Latinos will vote for a Republican need to take a look at Puerto Rico," he said.
Some observers thought Mr Santorum, a devout Catholic and opponent of abortion and gay marriage, might do well in the predominantly Roman Catholic territory.
But he angered many last week when he suggested Puerto Rico needed to make English its official language if it wanted to become the 51st US state.
The 3.7 million inhabitants of the Spanish-speaking Caribbean island - which is currently a self-governing US commonwealth territory - will vote in November in a statehood referendum.
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