The deal is worth $46 million total, with just over $24 million guaranteed, a source told ESPN.
Baldwin, 27, had been entering the final year of his current deal. He joined the Seahawks as an undrafted free agent in 2011 and had a monster season in '15, setting career highs in catches (78) and yards (1,069). Baldwin was tied for the league lead with 14 touchdowns.
Seattle coach Pete Carroll said earlier this month that the team was working with Baldwin's agent on a contract extension.
"It is a big deal. It is a very serious negotiation, as they all are," Carroll said at the time. "But he's done an incredible job for us and been a great teammate. And hopefully, eventually we'll get something worked out. I hope it happens. The intent is to get him signed and secured for a good while. So we'll see if we can get that done."
Baldwin, 27, said he wasn't worried about the talks.
"There's nothing to figure out," Baldwin said. "I'm playing football, each day as it comes by. And again, if I focus on the task at hand, all that other stuff will take care of itself. So I don't really have to think about it too much."
Information from ESPN's Adam Caplan was used in this report.