Arizona sent minor league infielder Zachary Walters to Washington for Marquis, who is 8-5 with a 3.95 ERA in 20 starts this season.
"I think he has a lot to bring to the table, not only on the mound," manager Kirk Gibson said before the Diamondbacks played the Los Angeles Dodgers. "I know that he's very dedicated and prepares well."
Marquis, who turns 33 in Aug. 21, was scheduled to start for the Nationals on Saturday night against the New York Mets. He goes from a team that was 17 games out in the NL East to a club that was four games off the pace in the NL West entering Saturday.
"I'm excited about it," Marquis said. "Obviously, going to Arizona they're in a good position to make a run for the playoffs, so it's nice to be part of a team, being wanted in that sense, that I'm a guy that can help that team."
The 21-year-old Walters hit .302 with nine homers and 56 RBIs in 97 games with Class-A South Bend this season.
"We fielded a lot of calls, and we felt this was the best deal player-wise and the financial package that we got back," Nationals general manager Mike Rizzo said.
Marquis agreed to a two-year contract with Washington in January 2010 and went 10-14 with a 4.82 ERA in 33 starts with the Nationals. He missed part of last year after undergoing surgery to remove bone chips from his right elbow.
"He's one of the hardest working guys I know," Rizzo said. "He worked really, really hard to get back to the Jason Marquis of old. It's really rewarding to us to be able to trade him and get some value back for him because he's worked so hard to get back to where he is."
Marquis also has pitched for Atlanta, St. Louis, Colorado and the Chicago Cubs in 12 major league seasons. He is expected to arrive in San Francisco either Sunday night or Monday, and he will throw a bullpen session before his scheduled Arizona debut on Wednesday against the World Series champion Giants.
Marquis is 5-3 with a 2.47 ERA in 12 career appearances against San Francisco.
"It definitely makes us stronger as a staff," Gibson said. "He's certainly a guy who will add to our rotation, and he makes us better. He's a veteran guy, a bulldog. He knows how to pitch and get our of situations. He's very durable -- and we've had a history with him, so that's interesting. I think it shows you that the guy plays the game. We know that."
On June 5 at Phoenix, Marquis was ejected in the sixth inning for hitting Justin Upton with a pitch, the fourth time he was plunked in that four-game series. Marquis did it following a warning from plate umpire Rod Drake, after Arizona's Ian Kennedy hit Jayson Werth.
"We don't have any bad blood. It's not a personal thing," Gibson said. "Everybody understands what goes on in a game. They really do. It's just part of the game. There's no bad blood, though. It might have been a situation then, but it's no big deal."
Starting pitcher Joe Saunders is one new teammate who is more than willing put that incident in his rearview mirror.
"He started that beanball game with Justin, so it's kind of interesting in that respect," Saunders said with a grin. "The funny thing about baseball is, you think about trading for a guy that you don't really like, so that's always an intangible. But in this game you have to let bygones be bygones and turn the page quickly -- because you never know when the guy you hate is going to be on your team. Now you have to like him, and he's your best friend."
"There's probably many different scenarios, but we're not there yet, so I don't have that figured out," Gibson said. "We'll have some decision to make on how it all shakes out."
Arizona also recalled reliever Ryan Cook from Triple-A Reno and placed left-hander Alberto Castillo on the disabled list with shoulder tendinitis. To make room on the 40-man roster, the Diamondbacks transferred shortstop Stephen Drew (broken right ankle) to the 60-day disabled list.
"Castillo had a sharp pain in his shoulder last night, so we're not going to push him at this point," Gibson said. "We'll be conservative with him and send him back home to see the doctor so that it doesn't turn into something serious."