SAN DIEGO — Pete Alonso had called it a must-win game. Give the Mets rookie credit for taking matters into his own hands.

With the Mets desperately needing a jolt, Alonso delivered with a mammoth two-run homer in the ninth inning Tuesday night that scored the go-ahead runs in his team’s 7-6 victory over the Padres at Petco Park.

Alonso’s 449-foot rocket off former Yankee Adam Warren hit the Western Metal Supply Co., in left field and culminated a 3-for-5 night in which Alonso drove in four runs, helping the Mets snap a four-game losing streak.

“Today was a real big character test for us and that was a really good team win,” Alonso said. “We had contributions from a lot of guys and I feel like we answered the bell today.”

Edwin Diaz created drama in the ninth by loading the bases after the Padres had already scored a run, but escaped with his ninth save in as many tries by striking out Eric Hosmer before retiring Hunter Renfroe.

The Mets (17-19) had scored six runs in their previous six games combined before breaking out with a 12-hit attack against the Padres.

“That’s the team that we have out there, the offense that we have,” manager Mickey Callaway said. “That’s what we did early [in the season]. Keep the train moving. As long as we keep on creating opportunities we’re going to score some runs.”

Brandon Nimmo snapped an 0-for-28 freefall with an RBI double in the seventh that tied it 5-5. Only Wilson Ramos’ lack of speed — the catcher was on first base — prevented the Mets from taking the lead on Nimmo’s double.

“I was just happy I found the barrel,” Nimmo said. “That is the first time in a while I had found the barrel.”

The Mets trailed 5-2 entering the inning, but Alonso delivered an RBI single and Michael Conforto’s sacrifice fly brought in another run. Amed Rosario’s double to put runners on second and third set up the inning.

Noah Syndergaard followed his best performance of the season — a shutout against the Reds — with a mediocre outing. The right-hander lasted six innings and allowed five runs, four of which were earned, on nine hits and one walk.

“I felt for the most part I battled out there with what I had available,” Syndergaard said. “What hurt me were the hanging sliders.”

Syndergaard fell into a three-run hole in the sixth on Hunter Renfroe’s RBI single off Robinson Cano’s glove — which easily could have been ruled an error. Jeff McNeil had missed on a diving catch on Hosmer’s ball leading off the inning that went for a double.

The Mets missed an opportunity to build a big lead against Padres starter Cal Quantrill early, leaving six runners on base in the first three innings. That included leaving the bases loaded in the third, when struggling Todd Frazier was retired to end the inning.

But in the first, the Mets had runners on the corners and one out when Ramos swung on 3-0 and grounded out to the pitcher.

Syndergaard hung an 0-2 slider in the fourth that Ty France hammered into the left-field seats for a two-run homer that put the Mets in a 4-2 hole. The previous batter, Renfroe, smashed a double off the left-field fence to start the rally.

Conforto kept the damage from possibly escalating by slamming into the fence as he caught Manny Machado’s shot to right field with two outs in the fifth. Conforto also had a strong throw to second that nailed France attempting to stretch a single leading off the second inning.
On the play France came off the bag and he was tagged out by Amed Rosario.

Franmil Reyes’ two-out solo homer in the third had tied it 2-2. The Padres scored an unearned run on the first when Rosario threw behind Syndergaard covering first trying to complete a double play. The error was Rosario’s 10th of the season.

The Mets ripped four straight hits to begin the game and take a 2-0 lead. Rosario and Alonso each had an RBI single in the inning after McNeil doubled leading off the game. Cano doubled for the 2,500th hit of his career, making him the third active player to reach that plateau, along with Albert Pujols and Miguel Cabrera.