It has been a long season for Giancarlo Stanton, who has been booed often, whose .205 average hovers just above the Mendoza line and who has blamed himself for both his struggles and a hamstring injury that cost him nearly seven weeks.

In the midst of a down and frustrating year, he allowed himself a few minutes to celebrate, and an appreciative Bronx crowd joined him.

Stanton became the fourth-fastest in MLB history to reach home run No. 400, smashing the milestone dinger in game 1,520 of his career.

The two-run, go-ahead shot in the sixth inning carried the Yankees over the Tigers in Tuesday’s 5-1 win.

“I didn’t have a number in mind when I first started this game, but it’s pretty cool to be here now,” said Stanton, who reached that number faster than anyone but Mark McGwire, Babe Ruth and Alex Rodriguez.

Stanton, who became the 10th player to accomplish the feat as a Yankee, took a curtain call and lifted his helmet to the fans, who stood and clapped in recognition.

Anthony Volpe used a timeout at the plate to give Stanton a bit more time to receive his applause. There have not been enough momentous home runs this season — he has hit 22 after smoking 31 in 110 games last year — but the 116.8 mph bullet allowed himself a brief moment to reflect.

“It’s definitely a cool little wake-up call, bigger picture,” said Stanton, who has three homers in his past five games. “Doesn’t change much in terms of how the year’s gone.

“But we do have a month left. So that’s all I could really focus or pay attention to.”

The homer helped make Gerrit Cole a winner, and the Yankees ace remembered back to early in their careers — when he was a Pirate and Stanton a Marlin — when the two talked during batting practice a day ahead of a Cole start. Stanton asked for a grooved fastball he promised he would take to right field as he sought to rediscover his stroke.

Cole obliged and Stanton “totally reneged,” Cole said, and blasted a home run to left.

“He just totally redeemed himself tonight, getting me that win,” Cole said.