Matthew Schaefer has put pen to paper.

As Schaefer walked out to the Citi Field mound to throw the first pitch at Monday’s Mets-Guardians game, the Islanders announced the defenseman has signed his three-year, entry-level deal.

Schaefer can play nine games with the Islanders this season without using up a year of his contract.

More likely, he will play the full season with the Isles, but he needs to win a job in training camp to make that official.