Washington clears $800,000 in salary-cap space before the trade deadline Monday by dealing one of the eight defencemen on its active roster. New Jersey gets a 23-year-old left shot on the blue line for its rebuilding efforts.
Siegenthaler is a restricted free agent after this season. The 2015 second-round pick from Switzerland has 13 points in 97 regular-season NHL games, all with the Capitals.
The third-round pick the Devils sent to the Capitals is Arizona's that they acquired by trading 2018 MVP Taylor Hall to the Coyotes last year.
New Jersey put veteran defenceman Sami Vatanen on waivers Sunday and could also trade Dmitry Kulikov and Ryan Murray before the deadline.