What a beautiful gesture by an Amazin’ guy.

A small act of defiance by Mets super-slugger Pete Alonso, destined to be named National League Rookie of the Year, brings him from Queens to the 9/11 Memorial & Museum in Lower Manhattan Tuesday.

The Mets first baseman is donating a pair of cleats he wore at Citi Field on the 18th anniversary of the terror attacks of 9/11, decorated to honor the fallen and the heroic first responders who gave so much, including their own lives, on that awful day.

He presented a pair of these custom-made shoes, each featuring a rendering of the New York City skyline, representing Pete’s adopted hometown, to every one of his teammates. And they wore them proudly, thumbing their noses at the brain trust running Major League Baseball.

It almost never happened.

MLB bosses had banned players from wearing hats and other clothing designed to commemorate anything, well, important — although they made an exception for the Houston Astros during the 50th anniversary of the first moon walk.

This made absolutely no sense — to Pete, or to practically anyone else. So Alonso, all of 24, took a gamble.

He failed to inform the powers-that-be that he and his teammates were taking matters into their own feet, donning the cleats to that day’s game, in which they beat the Arizona Diamondbacks 9-0, bosses be damned. He knew that he and the other Mets could face punishment, maybe suspensions, dressings-down or gigantic fines. But it was worth it.

Pete — everyone calls him “Polar Bear,’’ a nickname bestowed by teammate Todd Frazier — knew instinctively that this tiny risk paled in comparison to the actions of all those who made the ultimate sacrifice so that he might one day play a little game in the city where the Twin Towers once stood.

At a time when some athletes diss the American flag, here was a chance to say “thank you’’ to those who gave their lives and to all the bona fide heroes.

And they did.

Pete, I should point out, was 6 years old and living in Florida on Sept. 11, 2001. But Peter Morgan Alonso has got New York City in his blood.

His late paternal grandfather, Peter Conrad Alonso, emigrated from Spain through Ellis Island in the 1930s, served with the Americans in World War II, and settled in Queens as a rabid Brooklyn Dodgers fan.

His father, Peter Matthew Alonso, was 1 when the family moved to Ohio. Young Pete was born and raised in Tampa, Florida.

Pete said he’s certain “Grandpa’’ would be a Mets fan if he were alive today.

And what a year he’s had with the Amazin’s!

Alonso won the 2019 Home Run Derby, just ahead of the All-Star Game. (Another sign of his heart: He donated 10% of the $1 million he won to two charities — The Stephen Siller Tunnel to Towers Foundation and the Wounded Warrior Project).

Then he shattered the home-run record for a first-year player, hitting his 53rd dinger Saturday, and outstripping the previous record of 52 set in 2017 by then-rookie outfielder Aaron Judge of the New York Yankees.

His solid showing on and off the field just might help explain why red-faced baseball officials relented, and failed to punish a single Met for essentially proving on their soles that they have souls.

Sadly, the Mets did not make it to the playoffs this year. But Alonso proved that he’s a true patriot, a world-class New Yorker and a total mensch.

Here’s to you, Polar Bear! We’ll see you on the field next year.