MIAMI — Pete Alonso has got balls.

There is the ball from his first major league hit, which came last week during his debut in Washington. Then there is the ball from his first career home run, which was retrieved from behind the center field fence at Marlins Park, nearly 450 feet from home plate, earlier this week.

Alonso has plans for both souvenirs.

“They will definitely be cased up and put in my man-cave one day,” Alonso said before the Mets’ 6-5 victory over the Marlins Tuesday. He struck out looking as a pinch-hitter in the seventh inning.

In a whirlwind initial week on a major league roster, the 24-year-old Mets rookie first baseman managed to get many “firsts” out of the way. He was on the bench (another first) Tuesday night with a robust .412/.474/.765 slash line in his first five games.

The early breakout has validated everything the Mets heard about Alonso last season as he was hammering 36 homers in the minor leagues.

“Just the raw strength that he possesses,” Noah Syndergaard said when asked what has impressed him about Alonso. “He is a hard worker and student of the game and he’s always trying to be a sponge. He’s a great teammate to have and he’s starting to blossom into a beautiful butterfly.”

It was last Wednesday during a workout in Washington that manager Mickey Callaway gave Alonso the official word he had cracked the major league roster. In the blink of an eye, Alonso has become a celebrated figure in New York, even if he hasn’t played his first game yet at Citi Field.

That initiation will come Thursday in the home opener against the Nationals. It’s a safe bet that some of the loudest cheers in pregame introductions will be for Alonso.

“New York is a great place to succeed,” Syndergaard said. “It’s a very rewarding place to succeed. He may be a rookie, but he doesn’t carry himself as a rookie.”

Last September, Alonso participated in a skills camp for minor leaguers at Citi Field. He has never seen a game there and is relying on teammates this week to set the scene for him. Recently, Michael Conforto provided Alonso with a description.

“He told me that when Citi Field gets rocking you can feel the ground shake,” Alonso said. “There is no place to play in baseball that is better than that, so I am really pumped up and can’t wait.”

What “first” would he like to achieve next?

“Hopefully the first triple,” Alonso said.

The true test for Alonso is still ahead, as opposing pitchers make adjustments on him.

“I think he is going to hit with some power, but I do think he is going to start striking out more once they figure out how to pitch him,” a major league scout said. “He’s an interesting player and hopefully he will be a legit guy.”

After hitting a three-run homer in the ninth inning Monday night to break open a close game, Alonso entered the clubhouse to find his teammates waiting by the showers. He was put into a cart and smothered with eggs, barbecue sauce, beer, ketchup, mustard, mayonnaise and various others condiments and foods from the kitchenette.

As disgusting as the right of passage was, Alonso will never forget it.

“I had no idea that was going to happen, but it was a blast,” Alonso said. “It was a beer and food shower. That was fun.”