The Associated Press reported that the deal is worth $45 million for five years. Washington will make a formal announcement Monday at a 4 p.m. ET news conference.
Team president Stan Kasten and agent Brodie Van Wagenen have been working on a new deal for Zimmerman since March 1, the day Jim Bowden resigned as the club's general manager.
Kasten and Van Wagenen did not return phone messages.
When asked if a deal was close on Friday, Zimmerman said he was not aware of any negotiations going on between his agent and Kasten. In fact, Zimmerman said he did not want to negotiate a contract extension during the season.
"Talk to Stan and Brodie -- maybe they are doing something, but I'm playing baseball," Zimmerman said. "I haven't heard anything."
Zimmerman is considered by many to be the Nationals' best player. In his first full two years with Washington, the 24-year-old averaged 100.5 RBIs, but he missed 48 games last year because of a left labrum tear.
Entering Monday's action against the Braves, Zimmerman is hitting .275 with two home runs and 10 RBIs.
Acting general manager Mike Rizzo recently declined to comment on Zimmerman's contract status, but he said that the third baseman is the face of the franchise.
"He is a vital part of the franchise, both as the face of the franchise and one of its star players," Rizzo said.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.