The two sides were unable to reach a long-term agreement.
Pace, 28, practiced Tuesday and is expected to be ready for the opener Sept. 7 against the New York Giants.
Negotiations reached a stalemate in February, and the Rams placed made Pace their franchise player, thus inhibiting his unrestricted free agent status.
"It gives everybody a lift," said Coach Mike Martz said about Pace coming in. "He's been such an important part of our success in the years past that it's a big lift for everybody."
"It feels really good just to be back and to get out here and practice," Pace said.
Pace, a 6-7, 320-pounder, carried out his promise to hold out during training camp. He is responsible for helping to open holes on the left side for Marshall Faulk, the NFL's premier all-purpose running back.
Pace, one of the NFL's best, was the first overall pick in the 1997 draft out of Ohio State, has been selected to the last four Pro Bowls, and has started seven playoff games.