Haren joins a solid rotation that includes Stephen Strasburg, Jordan Zimmermann, Gio Gonzalez and Ross Detwiler, effectively replacing Edwin Jackson from the 2012 staff.
"[Haren is a] great athlete, outstanding stuff, gamer," manager Davey Johnson said. "He fits right in. I know he can swing the bat. If we get that finalized, he'll be a great addition."
General manager Mike Rizzo doesn't want to comment on Haren until the deal is finalized, but he is happy with the way the rotation is shaping up.
"We love our rotation," Rizzo said. "We feel that it gives us a chance to win every day."
Haren, a three-time All-Star, spent the last two-plus seasons with the Angels. He missed time in 2012 because of injuries to his back and hip, but he still made 30 starts and won 12 games.
The Angels declined Haren's 2013 option and bought him out for $3.5 million after trying to trade him to the Cubs last month for Carlos Marmol, a deal that was reportedly called off because Chicago did not like the medical reports on Haren's hip.
When healthy, the 32-year-old Haren brings innings and a lot of strikeouts. In 2009, for example, he struck out 223 in 229 1/3 innings, and he has a career rate of 7.6 strikeouts per nine innings.
Haren started his career with the Cardinals in 2003 before playing three seasons with the Athletics and 2 1/2 years with the D-backs, and he will bring a veteran presence to Washington's young pitching staff.
"I've got some young guys that act like veterans, and they pitched like veterans last year for me," Johnson said. "And a veteran like Dan Haren is just going to make things even better. I think he's a great addition. I've seen him pitch a lot over the years, and [he's an] intense competitor."
Having signed Haren, the Nats will not pursue free-agent right-hander Zack Greinke. Washington was also said to have been interested in Rays right-hander James Shields, but the two clubs never came close to making a deal.
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.