DeHaven, 58, is the third assistant to leave coach Bill Parcells' staff since the end of the 2006 season. Defensive coordinator Mike Zimmer took the same position with the Atlanta Falcons and quarterbacks coach David Lee moved to the University of Arkansas as offensive coordinator.
A 20-year NFL veteran, DeHaven has long been regarded as one of the league's top special teams coaches. In Seattle, he will replace Bob Casullo, who left earlier this week to become the tight ends coach for the Tampa Bay Buccaneers.
It had been rumored for the past week that DeHaven might depart the Cowboys after four seasons with the franchise.
After four years in the college ranks, DeHaven moved to the professional level in 1983, when he took a job with the New Jersey Generals of the USFL. In three years, DeHaven worked for three different USFL teams.
In 1987, he began a 13-year relationship with the Buffalo Bills, and his units were ranked at the best in the league twice during his tenure there. He worked three seasons for the San Francisco 49ers (2000-2002), before joining Parcells' staff in 2003.
Senior writer Len Pasquarelli covers the NFL for ESPN.com.