Coach Dick Jauron made the announcement after practice, saying Losman had clearly performed best during training camp to earn the No. 1 job in a three-way competition that included journeyman veteran Kelly Holcomb and free-agent addition Craig Nall.
"I told him he should feel good about it, he earned it," said Jauron, shortly after he informed the quarterbacks of his decision.
Jauron, who as recently as Monday declined to name his starter, said there was no point delaying the decision any longer with Buffalo preparing for its preseason finale at Detroit on Thursday and with the regular-season opener fast approaching on Sept. 10 at New England.
"We just wondered why there was any reason not to say it if we all thought it," Jauron said. "It gets him used to hearing it and getting the team used to hearing it. There it is."
The news did not come as a big surprise after Jauron named Losman the front-runner two weeks ago in training camp. Ever since, Losman has taken the majority of snaps in practice and started the past two preseason games.
"It's good for him to actually say it, but I've been getting kind of comfortable here the last couple of weeks," Losman said. "It's exciting times but it's time to really go to work now."
Losman was inconsistent in his first preseason start in a 44-31 loss to Cincinnati on Aug. 18. He went 7-of-11 for 134 yards passing and a touchdown, but committed three turnovers, including an interception returned for a touchdown.
Losman was more efficient in last weekend's 20-17 loss to Cleveland, going 15-of-22 for 184 yards passing with a touchdown and an interception. Overall this preseason, he's gone 37-of-57 for 462 yards passing with two TDs and two interceptions for a respectable 87.0 quarterback rating.
He struggled as a first-time starter last season. He was awarded the job in February -- a move that led the Bills to release veteran Drew Bledsoe.
Losman's status as No. 1 didn't last long, benched in favor of Holcomb after opening the season 1-3.
Holcomb went 2-2 in four starts before he was hurt in the second quarter against Kansas City, a game Losman rallied the Bills to a 14-3 win.
Losman, however, started and lost Buffalo's next four games and was replaced again by Holcomb, who went 1-2 to finish the season.
Holcomb had the better numbers, finishing with a higher completion percentage (67 percent to Losman's 50) and quarterback rating (85.6 to 64.9), and had 10 touchdowns to Losman's eight. Both threw eight interceptions.
Nall, the former Green Bay backup, was added to the mix when the Bills signed him in March. His development was hampered when he missed two weeks of training camp because of a hamstring injury.
Jauron added that he has not yet determined whether Holcomb or Nall will open the season as the primary backup.