Boston lost its last eight games, including getting swept in a season-ending three-game series by its archival New York Yankees, and ended with a 69-93 record. Only two AL teams had worse records.
"Our 2012 season was disappointing for many reasons," said Red Sox General Manager Ben Cherington. "No single issue is the reason and no single individual is to blame. We've been making personnel changes since August, and we will continue to do so as we build a contending club.
"With a historic number of injuries, Bobby was dealt a difficult hand. He did the best he could under seriously adverse circumstances and I am thankful to him."
Valentine, 62, was introduced as the team's manager last December. He'd previously managed the Texas Rangers (1985-92) and New York Mets (1996-2002), leading the Mets to National League pennant in 2000, when the New York Yankees beat the Mets four games-to-one in the World Series.
Valentine also had two stints leading Japan's Chiba Lotte Marines.
Counting the 2012 record with the Red Sox, Valentine has a major-league managerial record of 1,186-1,165.
The Red Sox dismissed Manager Terry Francona last year after the team suffered through a monumental September collapse.