Alonso finally gets his 200th home run

Pete Alonso hit another milestone Saturday, slamming his 200th career homer — and he became the fourth-fastest player to get to the mark.

“It was a really special moment,’’ Alonso said after the 7-4 loss to St. Louis at Citi Field.

He called getting to 200 homers so quickly “mind-boggling.”

Only Ryan Howard (658 games), Aaron Judge (671 games) and Ralph Kiner (706 games) got there faster, according to Elias.

“That’s a special group and that’s who he is,’’ Carlos Mendoza said. “He’s a special hitter.”

Alonso became just the fourth Met to reach the milestone, joining Darryl Strawberry, David Wright and Mike Piazza.

Mendoza noted that the blast was a shot out to right-center and that right fielder Lars Nootbar didn’t move on the play.

“You don’t see too many reactions like that from a right fielder,’’ the manager said. “He knew it was gone.”

Alonso is expected to be one of the key beneficiaries in J.D. Martinez’s arrival, with Martinez’s big bat directly behind him in the batting order.

He hopes Martinez can help, although he’s not worried about how opposing pitchers might alter their approach to him with Martinez in the ondeck circle.

“For me, the biggest thing is to not focus on that and fall into that trap because nothing is guaranteed,’’ Alonso said. “My whole goal is always to capitalize on mistakes. Whenever they make mistakes in the zone, I want to hit it hard between the lines.”

The theory, then, would be that Alonso is more likely to get more mistakes — and pitches to hit — because pitchers are wary of facing Martinez with another runner on base.

“I don’t know if that’s gonna happen,’’ said Alonso, who went hitless in four at-bats on Friday in his first game with Martinez behind him. “We’ll see. Hopefully.”

Regardless, Alonso said he won’t be focused on that.

“No matter what is going on around me, the plan is always for my focus to be on executing on what’s in front of me,’’ Alonso said. “If I’m not getting much to hit, my job is to take walks and have good at-bats.

And then put a good swing on it if they throw it over the good part of the plate and hit it hard between the lines.”

That’s what Martinez did in his first game with the Mets on Friday after just three rehab games following the back tightness that shut him down earlier this month.

Martinez is trying to spark an offense that has seen several players, including Brett Baty, Brandon Nimmo, Harrison Bader and DJ Stewart, all enter mini slumps.

Now, there’s Alonso and Martinez in the middle of the lineup. Alonso marveled at Martinez’s looking so good at the plate despite the lack of playing time leading into his debut.

“It’s funny, I remember his first day in camp, he was facing some pretty good arms and prospects and he had three or four doubles,” Alonso said. “The dude just rakes. J.D. just flat-out rakes. He’s shown that his entire career and that’s a skill that’s incredibly difficult to polish over the years and he’s done it. He’s one of the best.”