Coach Chip Kelly met with Foles and Michael Vick on Tuesday morning to explain his decision.
"Nick will be our starter," Kelly said. "Hopefully we get Mike back for a full week of practice, and he'll be the No. 2 guy this week."
Kelly said Foles will be the starter for the rest of the season, barring injury.
"I've always believed you need to have one quarterback," Kelly said. "I want our players to play from a desire to excel, not a fear of failure."
Vick won the starting quarterback job in competition with Foles during training camp. But after Vick pulled a hamstring on Oct. 6, Foles established himself as the starter. He tied an NFL record with seven touchdown passes in a game at Oakland and has presided over a three-game winning streak, leading the Eagles (6-5) into a first-place tie with the Dallas Cowboys in the NFC East.
"I feel the same," Foles said. "Nothing's changed in the way I am or who I am or the way I approach the game, how I am on the practice field, how I am in the locker room.
"I've always stayed true to who I am, and I've just got to continue to approach the game the same way every week."
Foles' passer rating of 152.8 in November was the highest rating for a full month in league history. So it's no wonder that Vick acknowledged during a recent radio appearance that he couldn't justify taking Foles out of the lineup.
"If we weren't winning games, then I'd be extremely upset within myself," Vick said Tuesday. "I wouldn't be upset with him.
"But your emotions change when you're not winning football games. We just feel good about what we're doing and we want to keep it in the right direction."
Kelly had postponed making any decision until Vick was healthy.
Coming off the Eagles' bye week, the 33-year-old Vick has been medically cleared to play. So Kelly sat down with Foles and Vick and made his decision official.
"They were both fine," Kelly said. "They understand the situation. Mike wants to contribute any way he can. That's the type of guy he is. Great teammate, great leader. He's been great for Nick, in terms of helping him develop. We've got a good situation. We've got two quarterbacks who can win games in the NFL, and you need both of them."
When Kelly named Vick the starter during training camp, he said the job was his for the season. Kelly didn't want his quarterback looking over his shoulder, and said that's the same deal for Foles now.
"Chip's a simple guy," Foles said. "I went in there, he told me the situation that I was the 1 and Mike was the 2, and let's go.
"It's really, it's not like drawn out like in movies, like it's a big thing -- you sit down, there are tears. He just told me the situation."
Kelly's decision will have long-term implications. Vick, who will be 34 next year, returned on a one-year contract with the expressed hope of earning a new deal with his performance.
Foles has two years remaining on his rookie contract. Matt Barkley, a fourth-round pick this year, is likely to be the No. 2 quarterback next season.
Depending on Foles' performance the rest of the way, Kelly could look to add a more mobile quarterback through the draft or other means in the offseason.