The 35-year-old Ratliff played 26 games for Minnesota and Detroit and appeared in 12 of the Pistons' 17 playoff games.
Terms of Ratliff's deal with the Sixers weren't disclosed.
"We're very fortunate to sign a player of Theo's caliber," general manager Ed Stefanski said. "He is a consummate professional, both on the court and off, who will provide us with additional depth in the frontcourt while also serving as a mentor to some of our younger players."
Ratliff has played for six teams during his 13 NBA seasons, including parts of four years with the Sixers from 1997-2001. In 705 career games, Ratliff has averaged 7.9 points, 6.1 rebounds and 2.6 blocks in 27 minutes.
Ratliff helped the Sixers to the best record in the Eastern Conference during the 2000-01 season, but was sent to Atlanta before the trade deadline in a multiplayer deal that brought Dikembe Mutombo to Philadelphia. Ratliff was an All-Star that season, but broke his wrist in early February, precipitating the trade.
Ratliff is one of seven players in NBA history to lead the league in blocks in at least three seasons (2000-01, 2002-03 and 2003-04). He ranks 18th in total blocks (1,855) and is seventh among active players.
The Sixers have been busy this summer after reaching the playoffs for the first time in three years. They signed two-time All-Star forward Elton Brand, added free-agent guards Royal Ivey and Kareem Rush last week and re-signed two of their most important players to long-term contracts, guard Louis Williams and forward Andre Iguodala.