With one swing — and the stare and bat flip that followed — Giancarlo Stanton breathed one big sigh of relief into Yankee Stadium.

The Yankees slugger had been booed as early as Opening Day and again in the first inning Monday as he remained hitless for an offense that was also off to a slow start. But by the fifth inning, he quieted those concerns as quickly as his 471-foot moonshot reached the terrace in left-center field.

Stanton turned boos into roars from the crowd of 9,008 when he crushed a grand slam to break open the game and send the Yankees to a 7-0 win over the Orioles on Monday night.

“The way he hits them, it’s just different. It’s like nothing else,” manager Aaron Boone said.

“He is a unicorn. That ball was just so pure on a night where it’s probably not flying great and just hits it through the stadium.”

Jordan Montgomery was sharp, tossing six shutout innings with seven strikeouts, mowing down an Orioles lineup that was coming off a sweep of the Red Sox in Boston. The Yankees pitching staff had been solid through the first three games, but had only one win to show for it because of a lack of run support.

The big bats finally began to break out, though, with Stanton leading the charge and Aaron Judge adding his first home run of the season — after Gary Sanchez had been responsible for the Yankees’ only two long balls through the first three games.

“We were kind of waiting on that one big hit with guys on base and big G delivered for us tonight,” said Judge, whose solo shot to the short porch in right field put the Yankees up 1-0 in the fourth inning. “People say it doesn’t weigh on you [waiting for the first home run], but it weighs on you sometimes. … When you get it off your back, now it’s time to go.”

Giancarlo Stanton watches his fifth-inning grand slam leave Yankee Stadium.Robert SaboStanton’s blast came with two outs in the fifth inning, snapping his 0-for-10 start. Aaron Hicks, who was in his own 1-for-14 rut to begin the season, had just drawn a bases-loaded walk to make it a 2-0 Yankees lead. Stanton then got a 92-mph fastball down the middle from Shawn Armstrong and turned on it, leaving no doubt with a 115.1 mph missile. He stared it down as it flew into the stands for the 6-0 lead, with the crowd in a frenzy.

“I try my best when I’m out there, so I can’t worry about all of that,” Stanton said of trying to win the fans over after their cold reaction to his tough start. “You guys [the media] like to make a big deal out of — I had [eight] at-bats. So if you throw that into two months, I don’t even think someone’s yawning about it.”

After starting the season 4-for-24 with runners in scoring position, the Yankees went 3-for-5 Monday, including an RBI single from DJ LeMahieu to pad the lead to 7-0 in the sixth inning.

Boone had said he didn’t want the Yankees to chase hits because of the cold start and they seemed to oblige, staying patient and drawing seven walks.

“Better at-bats all night long,” Boone said. “We had more opportunities, more competitive at-bats up and down the order. Hopefully that can be a little springboard for the offense.”

Of course, the loudest at-bats were reserved for Stanton and Judge.

“I feel like we’re still rising,” Judge said. “We still haven’t started clicking on all cylinders yet. It’s only been four games into the season. We still got quite a bit of time to get rolling and everyone get on track.”