Valentine, 61, returns to baseball after a decade away from the field, having managed twice in the majors and once in Japan.
"I am honored, I'm humbled and I'm pretty damn excited," Valentine said during the news conference at Fenway Park. "This day is a special day, and it's more than a special day. It's the beginning of a life that I think is going to extend beyond anything else that I thought of doing.
"The talent level and the players that we have in this organization, I think, is a gift to anyone. And I'm the receiver of that gift."
Valentine succeeds Terry Francona, who was fired after the Red Sox endured a monumental September collapse that cost them a spot in the playoffs.
"He (Valentine) is the right man at the right time for this particular team," Red Sox principal owner John Henry said. "We're set to win, we should have won last year. We're built to win. We thought, in the end, that Bobby was the person most capable of taking us to where we want to go in 2012 and 2013."