Wilmer Flores walked into Citi Field for the first time on Monday since signing as a free agent with the Diamondbacks in January.

And boy, did it feel different.

“I was coming in from the other side of the stadium, a new entrance,” Flores told The Post prior to the first game of a four-game series between the Mets and Diamondbacks. “This is all new for me. I’ve never been [in the visiting clubhouse] before, but I’ll see how it is out there [on the field].”

The infielder received a roaring standing ovation when he first stepped to the plate in the Mets’ 3-1 win over the Diamondbacks. He tipped his helmet to the fans and then greeted his old pal Jacob deGrom with a friendly wink before he grounded out to kick off the second inning.

But Flores got a chance to show the Mets just what they’re missing out on — after the team allowed him to leave as a free agent following last season — with a leadoff homer to bring Arizona within one run in the fifth. Flores spent the first six years in the majors with the Mets who signed him out of Venezuela in 2007.

The Diamondbacks picked up the 28-year-old knowing what a passionate player he was. Flores was reliable and versatile during his tenure with the Mets, recording a .262/.303/.424 slash line with 68 homers and 253 RBIs in 581 games. Last season, Flores hit .267/.319/.417 with 11 homers in 51 games. He signed a one-year deal with Arizona that included a team option for 2020.

“Yeah, definitely [was disappointing], definitely.” Flores said of leaving the Mets. “It’s just business. I’m doing what I love, just in another city. I’m still playing baseball, and it’s been great.”

Fans got a glimpse of Flores’ love for the Mets in 2015 when he wiped away tears in the middle of a game against the Padres just before the trade deadline after he was all but dealt to the Brewers in a trade that never actually ended up happening.

The next night he blasted a 12th-inning, walk-off homer to win an important game against the Nationals and the legend of Wilmer was born. Flores and the Mets reached the World Series that year.

Following Monday’s game, Arizona was 1 ¹/₂ games behind the Cubs for the second NL wild-card spot and 2 ¹/₂ games ahead of the Mets. Flores has played a huge part in the Diamondbacks’ success, hitting seven homers with 15 RBIs in his last 35 games. He missed nine weeks due to a right foot contusion, but jumped right back into the D-backs’ playoff push.

The biggest challenge transitioning to a new team, according to Flores, is how much he has missed his old teammates. He said it was tough to pick up and leave — though he’s managed to keep in touch with some of them, particularly deGrom, Juan Lagares, Michael Conforto and Steven Matz.

Now almost through his first season in the desert, Flores is enjoying his time with the Diamondbacks, saying he’s been happy where they are in the race for the postseason. But there was a reason he was welcomed back to Citi Field with a video montage of some of his Mets highlights before Monday night’s first pitch.

“New York is always going to be home and it’s good to be back,” he said.

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