Erat, 31, is a left-handed shot who has played right wing. Coach Adam Oates could move him to left wing on the first line opposite the right-handed Alex Ovechkin.
Erat has two years left with an annual cap hit of $4.5 million. He has four goals and 17 assists in 36 games this season for the Predators.
Forsberg was the Caps’ No. 1 pick in the 2012 draft, No. 11 overall. He’s projected as a top-six winger in the NHL.
Forsberg, 18, was considered the second-best prospect in the Washington organization, behind only Evgeny Kuznetsov.
The Caps also got 21-year-old center Michael Latta, a 2009 draft pick who has played in the American Hockey League for the Milwaukee Admirals. He will report to AHL Hershey.
This move signals general manager George McPhee’s desire to win now at the expense of the future.
“As players that’s our only priority is this year, right now and making the playoffs,” right wing Troy Brouwer said hours before the trade was announced.
“I hope to go all in because I don’t ever want to write off a season or anything like that,” defenseman Karl Alzner said.