"Always wanted to play in front of these fans", Marshall wrote on one of the posts. The person spoke to the AP on the condition of anonymity because the contract had not been officially signed.
Marshall racked up at least 1,000 yards in eight of his 12 seasons and has over 12,000 yards for his career. His representatives did not return messages seeking comment.
A native of Pittsburgh, Marshall was limited to just five games last season after incurring an ankle injury - and subsequently undergoing season-ending surgery - during a Week 5 game with the Los Angeles Chargers.
A league source later confirmed to the Times that Marshall - the 23rd-leading receiver in NFL history with 12,215 yards - will sign with Seattle. He was released by the Giants in mid-April and was due to make $5 million this season with NY.
Marshall played in 31 games for the Jets from 2015-2016, posting 1,502 receiving yards in 2015 and 788 the following year.
With the 2015 Jets, Marshall came agonizingly close to making the playoffs for the first time, but the team lost a do-or-die Week 17 game to miss the postseason despite a 10-6 record.
Seattle has made a priority this offseason of focusing on adding bigger receivers to its depth.
But if Marshall isn't healthy enough to be the No. 2 or 3 receiver behind Doug Baldwin and possibly Tyler Lockett, what would be the point of having him in a reserve role? Seattle also signed 6-foot-3 Jaron Brown.