The Devils also will receive a 2012 second-round draft pick from the Capitals.
Arnott, 36, waived his no-trade clause just before the 3 p.m. ET deadline.
"This is the hardest decision I've had to make, I think, in my career," Arnott said about waiving the clause, according to the Newark Star-Ledger. "It took me a while. It was hard to leave. It really was.
"With the roll we were on and the group of guys we have in the dressing room, it was hard."
Arnott had 13 goals for the Devils this season. He is a clutch playoff performer, with 66 points in 106 postseason games. He won a Stanley Cup with them in 2000, scoring the Cup-winning goal against the Dallas Stars.
"There weren't many ... centers available, and he was one that a lot of people wanted to have," Capitals general manager George McPhee said. "He's been the captain of an NHL team, he's won a Cup -- and those kinds of intangibles, I think, were something we wanted in the room."
He will provide secondary scoring and veteran experience for a Capitals team that was bounced in the first round of the playoffs last spring by eighth-seeded Montreal.
Steckel, who is signed through the 2012-13 season at an annual cap hit of $1.1 million, has evolved into a top-notch defensive forward who is a skilled face-off man and should thrive under Devils coach Jacques Lemaire.
"We felt that in order for us to trade Jason Arnott, we had to bring a player into our lineup that would not miss a beat with the type of team we had. It had to be a perfect fit because we don't want to lose anything in what has been happening," Devils general manager Lou Lamoriello said.
After a horrid start, the Devils -- behind Lemaire, who replaced John MacLean 33 games into the season -- have made a stunning rebound and are fighting for a playoff spot, just nine points out.
"The difficulty of this decision certainly comes into knowing what Jason's thoughts were, knowing how he felt, his honesty," Lamoriello said.
Wideman, 27, has one more year on his contract, worth $3.9 million. He had 33 points (nine goals, 24 assists) and 33 penalty minutes while playing in 61 games for the Panthers.
Green was injured earlier this month when he took a puck to the head in a game against the Penguins, and was hit in the head again, this time by Derek Stepan's shoulder check, in last week's game against the New York Rangers. He has missed seven of the past nine games. McPhee said Monday that Green will be sidelined "at least a couple of weeks, maybe longer."
Poti has been battling a groin injury, and has played in only 21 games this season. The last game he played in was Jan. 12. There is no timetable for his return.
Hauswirth, 23, has 10 goals and four assists in 37 games with the South Carolina Stingrays of the ECHL this season.
Information from ESPN.com's Scott Burnside and Pierre LeBrun was used in this report.