Seraphin gets the Knicks' room exception of one year and $2.8 million.
The room exception carries a value of $2.8 million and is the smallest of three mid-level exceptions (standard and taxpayer being the others) that teams with cap room may use to sign players.
Seraphin averaged 6.6 points and 3.6 rebounds in 79 games with the Washington Wizards last season.
The move continues the makeover New York has undergone with its big men this offseason, including the signing of Robin Lopez.
New York hopes that the new free-agent additions can improve the club's rebounding and make an impact on defense. The Knicks had the third-worst rebounding differential in the NBA last season and ranked 28th in defensive efficiency.
Seraphin is the eighth new player added by team president Phil Jackson in the offseason.
ESPN.com's Ian Begley contributed to this report.