Coach Steve Spagnuolo announced the decision Saturday, saying Bradford had earned the job.
"We're confident. The staff's confident. The team's confident," Spagnuolo said. "It's well deserved."
The game is Sept. 12. A.J. Feeley, whose injury gave Bradford the chance to take plenty of snaps in the preseason, and Thad Lewis are the backups.
Bradford won the 2008 Heisman Trophy when he was at Oklahoma. He signed a six-year, $78 million contract with $50 million in guaranteed money -- the most ever in the NFL -- and then went out and played well in the preseason against the likes of New England and Baltimore.
"I feel like coach has given me a great opportunity to start next week against Arizona," Bradford said. "I realize there is a lot of responsibility that comes with that opportunity."
While the low-key Bradford did manage to smile when talking, he said he is ready.
"Definitely. I'm a competitor," Bradford said. "I sat on the bench last year with an injury and it's something that I did not enjoy at all. Just to have the opportunity to play football again, it's really something that I look forward to."
In his two preseason starts, Bradford was 21 for 28 for 257 yards and three touchdowns. For the preseason, Bradford completed 33 of 55 passes for 338 yards with no interceptions.
Bradford likely sealed the deal with his performance last Thursday against Baltimore in the preseason finale. Bradford's opening drive set the tone for a 27-21 victory that gave the Rams a 3-1 preseason finish for the second straight season. He was 6 for 6 for 68 yards and a touchdown in his one series, one week after a two-TD effort.
"It was evident in these last three games that he had pretty good control of what we're doing," Spagnuolo said. "The team kind of rallies around him."
The 33-year-old Feeley went down with an injured right thumb in the Rams' second preseason game at Cleveland and that put Bradford on the fast track to the starter's job. Feeley has only 15 career starts in 10 seasons and was signed by St. Louis as a free agent in the offseason.
"I think we're very, very fortunate to have A.J. Feeley in this particular role as a backup quarterback," Spagnuolo said. "He's a seasoned vet who knows the ins and outs and he can feed that to Sam as he goes along.
"I don't think anybody in this situation coming in as a rookie quarterback is ready for everything. I'm sure these defensive coordinators will fire things at him."
Lewis, who played at Duke, beat out Keith Null, a sixth-round pick last year who started the final four games of the Rams' 1-15 season.
Bradford threw just 69 passes last season at Oklahoma. He hurt his shoulder in the season opener against Brigham Young. He returned three weeks later and played against Baylor before hurting the shoulder the following week against Texas and he did not play again.
Still, that did not keep Bradford from dreaming about being the No. 1 pick in the draft.
"Yeah, I think when you can imagine things like that, that's really what motivated me to work so hard to get myself back in this position," Bradford said. "I think I did work extremely hard in the offseason rehabbing my shoulder. Now that I have this opportunity, I want to make the most of it."