Belliard, 34, was hitting only .247 for the Nationals, with a .297 on-base percentage and five homers in 186 at-bats. However, he's heated up over the last few weeks, batting .367 in August, with three homers, a .406 on-base percentage and a .600 slugging percentage.
"He gives us some offense," manager Joe Torre said. "He can play around the infield, probably all positions but shortstop. It gives me the ability to spell these guys. He gives us a little more depth with experience coming off the bench."
He is primarily a second baseman, but has played third base and first as well this year. For the Dodgers, he will back up Orlando Hudson, Casey Blake and James Loney, and will provide a bat off the bench. The Dodgers will have to make a roster move to open a spot for him on Monday.
"It's a compliment," Belliard said, after the Cardinals beat the Nationals 2-1 in St. Louis to complete a three-game sweep. "Now I have a chance to win another one. I helped the Cardinals in 2006 with my defense and some lucky hits that I got in the playoffs. Hopefully I can do that in LA. That's why they traded for me."
Belliard also has postseason experience. He was the starting second baseman on the 2006 Cardinals team that won the World Series and hit .240 in 14 postseason games.
The Cardinals got Belliard in July 2006. During a playoff series against San Diego, he went 6 for 13. He went 0 for 12 in three World Series games.
Nationals manager Jim Riggleman wasn't surprised that Belliard drew the interest of a contending team.
"We've played pretty good baseball since the All-Star break, and he's been a part of that," Riggleman said. "On the other side of it, we get to keep an infusion of young players coming in and Ronnie gets to go play in a pennant race. Ronnie's play since the All-Star break has been noticed by scouts and so forth, and they identified him as a guy who could help them down the stretch."
Garcia is a 22-year-old righthander from the Dominican Republic. He is 5-3, with five saves, in 34 relief outings for Great Lake of the Class A Midwest League.
Jayson Stark is a senior baseball writer for ESPN.com. Information from The Associated Press was used in this report.