With 88 percent of the vote counted, CNN reported Romney grabbed 39 percent to 30 percent for Sen. John McCain of Arizona and 16 percent for Mike Huckabee, a former Arkansas governor.
After being declared the winner, Romney told his cheering supporters it was "a victory of optimism over Washington-style pessimism," the Detroit Free Press reported. Then he asked the affirming crowd, "Is Washington, D.C., broken? Can it be fixed? Are we the team that's going to get the job done?"
The closely watched race Tuesday was seen as a must-win for Romney, who was born in the state and the son of a popular three-term Michigan governor, The Detroit News reported.
McCain also has significant ties to the state, having won the Michigan primary in his 2000 unsuccessful presidential bid.
Going into Tuesday's election, Romney and McCain were in a neck-and-neck race. Candidates were hoping for a win in Michigan to provide their campaigns momentum going into the pivotal South Carolina primary in just four days.