Devils general manager Lou Lamoriello described the deals as multiyear, but he did not disclose details.
Niedermayer, Arnott and their agent, Don Meehan, were not immediately available for comment.
Lamoriello met with Meehan and the players last Sunday in Toronto.
Meehan called Lamoriello on Sunday morning to tell him the players had accepted the latest offer.
"It's unfortunate it took this long but they will be here more than a year," Lamoriello said.
Arnott and Niedermayer will take physicals on Monday and they are expected to practice with the team. They will accompany the team to the West coast for a road trip which starts on Wednesday in Anaheim, Calif. against the Mighty Ducks.
"Right now this is a positive," Lamoriello said in a telephone interview. "They are here and I'm excited."
Niedermayer, who gained notoriety for being suspended for 10 games late last season for hitting Florida's Peter Worrell over the head with a stick, had sought a $13.5 million, three-year contract.
Niedermayer made $3.25 million last season. Lamoriello had offered a three-year contract which would have worth $4 million annually if Niedermayer met incentives for selections to the All Star team and postseason honors.
Arnott, who scored the Stanley Cup clinching goal against Dallas in June, had been offered a two-year contract worth $5.1 million. He made $1.8 million in each of the last two seasons.
When the players rejected those offers, Lamoriello said he would deduct portions of their salaries for each day of the regular season they missed.
The return of Niedermayer and Arnott is welcome news to the Devils, who have posted an 8-8-3 record. They recently ended a six-game losing and seven-game winless streak with consecutive overtime wins.
The losing skid was the team's longest since 1991 and the winless slide was the longest since 1992.
"This is good for our team," Devils captain Scott Stevens said in a telephone interview. "They are important people and I'm glad it's over with."
Forward Randy McKay echoed Stevens' thoughts.
"We need a little more consistency and they will help with that," McKay said.
Arnott had 22 goals and 34 assists in the regular season. The rugged eight-year veteran stepped it up in th playoffs, collecting eight goals and 12 assists in 23 playoff game. The big goal came in Game 6 of the finals against the Stars when he scored at 8:20 of the second overtime for a 2-1 win that gave New Jersey its second cup since 1995.
Niedermayer had 7 goals and 31 assists in 71 regular-season games. He had five goals and two assists in 22 playoffs game, missing the opening game of the playoffs because of the Worrell-related suspension.
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