Terms of the deal were not disclosed.
"He's just a veteran presence, a guy that's had a lot of success in the league," Chargers offensive coordinator Joe Lombardi said Wednesday. "We thought that he would add some value to our running back room. So nothing against the guys that we have -- we just thought he was a good player that surprisingly became available."
However, depth at running back has been an ongoing concern since last season.
But Spiller has been dealing with an ankle injury since Week 2 of the preseason, leaving his status for the Sept. 11 opener against the Las Vegas Raiders uncertain, and Rountree was waived to make room for Michel.
Late in training camp, Lombardi said none of the backup candidates had separated from the competition.
Lombardi described the 27-year-old Michel on Wednesday as a "good runner" and "every-down back."
"We think he is going to add some value," Lombardi said.
A former first-round draft pick by the New England Patriots, the 27-year-old Michel spent last season with the Super Bowl champion Los Angeles Rams, who acquired him from the Patriots in a preseason trade.
Michel took over as starter in Week 13 and played a key role in helping the Rams emerge from a three-game losing streak to go on a Super Bowl run. In the final six games of the regular season, Michel rushed for 540 yards and three touchdowns.
Michel signed a one-year contract with the Dolphins this offseason but was the odd man out in a crowded backfield. The Dolphins have Chase Edmonds, Raheem Mostert, Salvon Ahmed and Myles Gaskin on their roster at running back, as well as fullback Alec Ingold.
Michel is a two-time Super Bowl champion and has rushed for 3,137 yards and 18 touchdowns in four NFL seasons.
ESPN's Marcel Louis-Jacques contributed to this report.