The Seahawks have yet to confirm the move, but ESPN cited multiple sources as saying that the deal is for one year. Adam Schefter added that Marshall's contract could be worth up to $2 million if he reaches incentives.
Marshall posted multiple photos of himself wearing a Seahawks jersey on his Instagram account.
"Always wanted to play in front of these fans.. . @seahawks," the 34-year-old Marshall wrote.
Marshall visited with the Seahawks earlier this month after being released by the New York Giants in April with a failed physical designation.
A six-time Pro Bowl selection, Marshall recorded just 18 receptions for 154 yards and zero touchdowns over five games with the Giants before an ankle injury ended his season.
The Seahawks' passing game took a hit when tight end Jimmy Graham and wide receiver Paul Richardson departed in free agency this offseason.
The 6-foot-7 Graham, who joined the Green Bay Packers, hauled in 10 touchdown passes in 2017 while Richardson set career highs in receptions (44), receiving yards (703) and touchdowns (six) last season before heading to the Washington Redskins.
Marshall's production began to decline in 2016, when he had 59 catches for 788 yards with the New York Jets.
He has had more than 100 receptions six times in his career and more than 1,000 receiving yards eight times, including 1,502 in 2015 with the Jets.