Halak was part of a multi-player deal last week with St. Louis, one which saw long-time Buffalo goalie Ryan Miller head to the Blues.
The Slovakian Halak moves now to the Capitals along with a third-round pick in 2015, in exchange for goalie Michal Neuvirth and veteran defenceman Rostislav Klesla.
Halak was in his fourth season with the Blues after spending his first four years with Montreal, including a 2009-10 campaign in which he stoned Alex Ovechkin and the Capitals in a seven-game playoff series.
He has a 24-9-4 record with a 2.23 goals-against average, a .917 save percentage and four shutouts this season.
Halak and Brian Elliott combined to win the William Jennings Trophy for top goals-against average in the league in 2011-12.
An unrestricted free agent this summer, he'll form a tandem in Washington with Braden Holtby.
Neuvirth was Washington's No. 1 goaltender in 2010-11, but injuries and the emergence of Holtby saw him fall out of favour.
Neuvirth, who turns 26 later this month, has one year left on a contract that next season will represent a salary-cap hit of $2.5 million US.
The Czech Republic native has appeared in 134 NHL regular-season games, going 59-41-13 with seven shutouts, a .910 save percentage and a 2.67 goals-against average.
Like Halak, Klesla has been traded twice during the days before the current deadline.
He was picked up by Washington from Phoenix in a multi-player deal on Tuesday. In 659 career games with the Coyotes and Columbus Blue Jackets, Klesla has scored 48 goals and 111 assists, with 620 penalty minutesSuggest a correction