One of the more appealing Plan Bs if the Mets were not to re-sign Pete Alonso is off the market already.

First baseman Josh Naylor agreed to a five-year contract to return to the Mariners, The Post’s Jon Heyman confirmed Sunday night, in the first major signing of the MLB offseason. The financial terms were not yet disclosed.

Naylor, 28, was acquired by Seattle from the Diamondbacks at the trade deadline and boasted an .831 OPS with nine homers and 19 steals in 54 games while batting .299.

It was part of a superb all-around year that included 20 homers and 92 RBIs

Mariners first baseman Josh Naylor (12) reacts after hitting a home run in the sixth inning against the Toronto Blue Jays during Game 6 of the ALCS. IMAGN IMAGES via Reuters Connect

Naylor helped the Mariners win the AL W

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