Sproles had been scheduled to become an unrestricted free agent. The Chargers tendered him a one-year contract offer of approximately $6.62 million, the average of the top five salaried running backs from 2008.
He made $1.1 million in base pay last year.
General manager A.J. Smith said the team plans to begin speaking with Sproles' agent, Gary Wichard, about a long-term deal.
Sproles can still negotiate with other teams, but his options are limited. If he receives an offer, the Chargers can either match it or receive two first-round draft choices as compensation, which would be a heavy price tag for a player who hasn't been an every-down back.
Smith also shot down speculation that putting the franchise tag on Sproles means the end of LaDainian Tomlinson's record-setting tenure in San Diego.
"This has nothing to do with L.T.," Smith said. "Our intention is to pursue L.T. We assume that at the combine it will pick up speed and there will be some substance to it when we return."
The Chargers are expected to try to renegotiate Tomlinson's contract. The star running back, limited by toe and groin injuries in 2008, will have a salary cap number of approximately $8.8 million next season. The Chargers would like to reduce that number so they can sign other players to long-term extensions.
Sproles didn't immediately return an e-mail seeking comment.
Wichard said he knew the franchise designation was coming. Although Sproles can't find out his fair-market value, Wichard said he and his client felt "it's kind of an honor."
"I think that putting the franchise tag on Darren certainly shows where the San Diego Chargers feel his value is," Wichard said. "I'm sure it'll be worked out between the team and I. Putting the franchise tag on him gives them breathing room to get other things taken care of. They know that if Darren goes out in free agency, it would be a different story."
Tomlinson said he was pleased for Sproles.
"He's obviously earned it," Tomlinson told The Associated Press at the San Diego Hall of Champions' Salute to the Champions dinner, which he attended to see Marshall Faulk inducted into the Breitbard Hall of Fame. "Players set out to prove that they can play in this league and he's definitely proved that he can play here, and he's earned the money."
Asked about his own contract situation, Tomlinson said: "I'm going to defer to next week. Then I'll know a little bit more."
Sproles said toward the end of last season that he'd like to remain in San Diego but that he'd also like to explore the market.
Smith said that forced the Chargers to make a decision, since Thursday is the deadline for designating franchise and transition players.
"We met and discussed the possibilities, which were many," Smith said. "We'd like for him to be here long term. The deadline has come, and we either let him go to the market or we franchise him. We felt a little uncomfortable with, 'Let's just be one of 32 and submit years and money and hope for the best,' or we franchise him, and he'll be here, and he'll be here at a set price."
With Tomlinson sidelined by a groin injury, Sproles stepped up and had a huge performance in a 23-17 win against Indianapolis in a wild-card playoff game. He gained 328 all-purpose yards and scored two touchdowns, including the winning 22-yard scamper in overtime.
He was bottled up the next week, however, as the Chargers lost 35-24 at Pittsburgh.