A dog from the city pound died of possible heat exhaustion while being driven to his forever home during the hottest weekend the Big Apple has seen in nearly a decade, The Post has learned.

Don — a 2-year-old pit bull who’d been rescued from Brooklyn’s Animal Care Center — was headed to Rescue Dogs Rescue Soldiers in Cherry Valley outside of Albany Saturday afternoon when the ACC transporter stopped at a gas station 10 minutes from the destination and found the pup dead in its cage, the rescue’s president Liz Keller told The Post.

“It was such a shock, no one was thinking clearly, we were all crying. It’s a day where everyone’s excited and happy, the dogs can’t wait to get out of the van,” Keller said, breaking down into tears.

“I said, ‘Open the doors,’ so I could at least see him, maybe we could do CPR, but he was gone and had been gone for a while because the body was really hard, it was very stiff, so I’m assuming he passed away very quick into the transport.”

Keller said it looked like Don had been “in distress” before his death because the cage was covered in feces and his mouth was “wide open like an alligator.”

Don left Brooklyn around 9 a.m. for the four-hour ride in the midst of the worst heat wave the city has seen since 2011, with heat indexes as high as 110 degrees on Saturday.

On Friday, the city’s Commissioner of Health Dr. Oxiris Barbot, whose department oversees the ACC, made numerous warnings alongside Mayor de Blasio that the conditions could be deadly for people and their pets.

Keller said that when the transporter arrived at the shelter, she was distraught and believed the high temperature was behind Don’s death.

“The transporter got out of the van and said, ‘I think he died of heat stroke because when I was driving I realized the air conditioner was on low,'” Keller explained.

“He was in the back of the van where the direct sun hits the vehicle.”

Another dog who was traveling with Don named Bambino was also in bad shape when the transporter arrived and was showing signs of dehydration.

“He was pale, his color wasn’t great but she’d already upped the air [by then] and he was cooling off,” Keller said.

Bambino “was in the side compartment and probably didn’t get as hot” because there wasn’t direct sunlight, she added. She said Bambino is now doing fine after he got inside and drank some water.

Keller, who used to work for ACC and the ASPCA, and has been working with dogs for decades, said there was no thermostat in the van and the transporter was “new” and “very young.”

“How do you use a van that doesn’t have a thermostat on the hottest day of the year?” Keller railed.

“I just don’t understand why that wasn’t a priority.”

A disabled man in Massachusetts had put in an application for Don and was eager to adopt him after he’d had some time to adjust to life outside of the pound. Keller said the adopter still hasn’t heard the news.

“He was really excited about it and I have to talk to him today,” Keller said.

“I’m really dreading that.”

Keller said Don’s cause of death was ruled inconclusive by the ACC. But she’s wary of the embattled agency, which is facing a probe from the state Attorney General over animal cruelty allegations.

Barbot’s office declined to comment. In a statement, ACC said only: “We are deeply saddened by this loss and are conducting a thorough investigation into the incident including a full examination of Don as well as the transport vehicle.”