Former Minneosta Gov. Arne Carlson made his announcement at the State Capitol before a roomful of Obama supporters.
Carlson said Obama is better equipped than John McCain to handle the struggling economy, foreign policy and to make the nation more energy independent.
Carlson said he was a strong supporter of John McCain during the primary season, but he became unhappy with the direction the campaign has taken since McCain accepted his party's nomination at the Republican Convention.
Carlson said Obama proved himself during the primary season.
"He [Barack Obama] went through a very grueling campaign, probably one of the most grueling in American history, and he had a remarkable resolve and an extraordinary ability to brush aside the criticism and stay focused. I truly admire that," said Carlson.
Carlson has had a frosty relationship with many Republican activists, and he said he was not approached to endorse McCain or asked to say neutral. Minnesota's current GOP Gov. Tim Pawlenty is a strong McCain supporter.