At 6-foot-2 and 217 pounds, Weber gives the NHL-leading Capitals another left-handed-shooting player and some insurance on the back end.
"Mike is a stay-at-home defenseman, who plays a physical game and is respected by his teammates," general manager Brian MacLellan said. "We felt it was important to add depth to our blue line by adding another quality veteran defenseman."
Weber, 28, has a goal and four assists and has averaged almost 16 minutes a game for Buffalo this season. He has blocked 666 shots since entering the league in 2007-08.
Buffalo retained half of Weber's salary and his $1.67 million cap hit as part of the deal.
"It's difficult to trade someone that has been a part of the organization for 10 years, but I think this is a good opportunity for the player and for our organization," Sabres general manager Tim Murray said.
Weber was a second-round pick of the Sabres in 2006 and has 53 points in 341 NHL games. He should be a third-pairing defenseman for Washington, which is first in the league by nine points over the Chicago Blackhawks.
Last week, Capitals coach Barry Trotz acknowledged that deadline trades can change players' roles but didn't seem worried about whether adding a player would disrupt team chemistry.
"Is it a part of the consideration? Yeah," Trotz said. "But if we find the right piece, we'll add it. If it can put us over the top and give us a huge advantage, we'll forgo the chemistry a little bit and go for it."