Vokoun, whose negotiating rights were traded for a seventh-round pick in the upcoming NHL draft, will make US$4 million with Pittsburgh.
The 35-year-old goalie, who waived a no-movement clause in order for the trade to happen, was set to become an unrestricted free agent July 1.
The move gives Pittsburgh an established backup to starter Marc-Andre Fleury, who tied his career high by appearing in 67 games last season.
Vokoun finished the regular season 25-17-2 with a 2.51 goals-against average and a .917 save percentage after signing a one-year deal.
He suffered a serious groin injury in March and lost the starting job to rookie Braden Holtby, who led the Capitals to the second round of the playoffs.
Vokoun has 287 career wins along with a 2.55 GAA and .917 save percentage in 680 career games for Montreal, Nashville, Florida and Washington.