Pete Alonso trusted his gut after his opening at-bat on Tuesday night against the Cubs, but an umpire told him to hold up.

After battling against Chicago starter Jameson Taillon in the first inning, Alonso smacked a 3-2 pitch 419 feet into center field, which bounced high off the wall as Alonso stopped at second for what at first seemed like an RBI double that scored Jeff McNeil from first. 

“It was just a strange at-bat,’’ Alonso said after the Mets’ 3-2 loss. “The reflection from the sun was glaring in my eyes from the glass in center. I didn’t get a good look at the ball and saw Cody [Bellinger] field the ball in center and said, ‘There’s a double.’ But as soon as I saw the replay, I said, ‘OK [it’s a homer].”

But after a replay of his hit was shown on Citi Field’s center field scoreboard, Alonso started heading home as the ball clearly cleared the orange line.

As he rounded third base, ready to high-five coach Joey Cora, he was stopped by the third-base umpire Ramon De Jesus, telling him to go back to second base because the play was still under official review.

Alonso threw up his hands in disbelief and jogged back to second base, only to turn around again to run home to complete what was eventually ruled a home run — his 34th of the year, giving the Mets a 2-0 lead.

“I guess it’s protocol for the umpires or whatever [to tell him to go back until it was official],” Alonso said. “But yeah, happy it went over the fence.”

It marked Alonso’s third home run in his last five at-bats after he went deep against the Cubs twice on Monday night.

In the series opener, he tallied six RBIs during his 18th career multi-homer game — third in franchise history behind Darryl Strawberry (22) and David Wright (21).