Happy the Elephant, the beloved Bronx Zoo icon who proved the incredible intelligence of her species and was the center of a “human” rights case, was euthanized Tuesday.

The 55-year-old Asian elephant was put to sleep at the Bronx Zoo, where she lived for almost half a century, following a rapid decline in her health in recent weeks.

“She died peacefully surrounded by the keepers, curators, and veterinarians who have cared for her; some for more than 30 years. Their longstanding, deep relationships were invaluable to Happy throughout her life,” Craig Piper, Interim Bronx Zoo Director and Executive Vice President for Wildlife Conservation Society Zoos and Aquarium, said Thursday.

“Making the decision to euthanize an animal is one of the hardest and most heartbreaking responsibilities in animal care, requiring immense compassion and love for the animal.”

Zoo officials say Happy’s age-related conditions accelerated in recent weeks, and she showed signs of a falloff in kidney and liver function.

A necropsy performed Wednesday showed that the beloved pachyderm had been living with several large inoperable uterine tumors and arthritis, which the zoo’s veterinarians believed also contributed to her poor health.

The elderly elephant had been living in the zoo’s hospice care for nearly a month, which meant she was given free rein of the elephant barn and outdoor yard spaces, which she previously had to alternately share with other elephants at different times.

“One of the most meaningful aspects of this difficult day was the opportunity for Happy’s keepers, supervisors, curators, and veterinarians to spend time with her both before and after she was euthanized. Many staff members formed deep and lasting bonds with Happy that were invaluable to her care throughout her life. She died peacefully surrounded by the people who loved and cared for her most,” Piper wrote in an internal memo to staff obtained by The Post.

“Caring for aging animals and making end-of-life decisions are among the hardest responsibilities in our profession, and our teams managed every step of this process with extraordinary care and respect for Happy.”

Happy first came to the Bronx Zoo in 1977.

She was born in the wild of Thailand in 1971 and captured along with six other calves — each of which was named for the dwarves in Snow White — and sold to the now-defunct Lion Country Safari in California.

She lived with other elephants at the zoo until her last partner died in 2006. Happy continued to share the outdoor exhibit with Patty and Maxine, but was separated by a barrier for the last two decades because Happy and Patty did not get along.

The trio was still able to touch, smell and hear each other as they moved from space to space as they chose.

Happy was a beloved fixture at the zoo and was known for stashing watermelon and strawberries inside her ear to save for later, and for raising her trunk as a greeting to her keepers, the zoo said.

Happy made history in 2005 when she proved to researchers that elephants can recognize themselves in a mirror — a sign of self-awareness seen in only a few other species.

That storied intelligence placed her in the center of a 2018 lawsuit by animal activists who tried to have Happy declared a “person.”

The lawsuit by the Nonhuman Rights Project, which claimed Happy was not being properly cared for nor had adequate space at the Bronx Zoo, was shot down by New York’s top court, but the activists continued to rally against Happy’s conditions in the final years of her life.

In a statement Thursday, the Nonhuman Rights Project alleged that the “Bronx Zoo and the Wildlife Conservation Society failed Happy.”

“The Wildlife Conservation Society’s Bronx Zoo kept Happy in captivity in conditions that did not meet her complex needs since 1977, and in solitary confinement for 20 years. As a result, she suffered significant physical and psychological harm,” Christopher Berry, executive director of NhRP, said.

“Happy lived at first with her companions, Grumpy and Sammi, but was kept in solitary confinement by the zoo after their deaths. She was forced to endure a near lifetime of chronic stress and trauma in the zoo’s tiny, barren exhibit.”

The group called for Patty to be released to an accredited elephant sanctuary.

The 57-year-old Asian elephant is the last at the zoo, following a 20-year-old decision by the Wildlife Conservation Society to stop acquiring pachyderms.