Webb's decision was not a big surprise, The Washington Post reports.
Former Sen. George Allen, whom Webb unseated in 2006, is running to regain the seat, along with other Republicans. Democrats are hoping to persuade former Gov. Tim Kaine to run, but he has not announced his plans.
"It has been a great and continuing privilege to serve in the United States Senate," Webb said. "We have given our post-9/11 veterans the best GI Bill since World War II, we have taken the lead in reforming our criminal justice system, we have led the way toward stronger relations in East and Southeast Asia and we have been a strong voice in calling on China to act more responsibly in the world community.
"However," he added, "after much thought and consideration I have decided to return to the private sector, where I have spent most of my professional life, and will not seek re-election in 2012. … I have every intention of remaining involved in the issues that affect the well-being and the future of our country."