Patrick, 54, had 48 percent of the vote to Baker's 41 percent, with 95 percent of precincts reporting. Independent Timothy Cahill was third with 8 percent and Green-Rainbow Party candidate Jill Stein had 1 percent.
Baker, 54, spent 10 years as chief executive officer of Harvard Pilgrim Health Care, a non-profit health-benefits company.
When New York Gov. David Paterson leaves office at the end of the year, Patrick will be the nation's only black governor.
U.S. Rep. Barney Frank, D-Mass., was re-elected to a 16th term, easily defeating a Republican challenge from political neophyte Sean Bielat.
Frank, 70, chairman of the House Financial Services Committee and co-author of the controversial financial-regulation reform law, defeated Bielat by 11 percentage points, a wider victory margin than some polls had suggested.
Bielat, 35, a former U.S. Marine, did not say in his concession speech if he would run for office again, but said, "This isn't going to end tonight."
"This is still about a hope for a brighter tomorrow, for a different future for this country," The Boston Globe quoted him as saying.