Retired NYPD Chief of Detectives John Keenan, who stormed Normandy, fought in the Battle of the Bulge and secured a confession from the notorious Son of Sam serial killer has died, police officials announced Friday.

Keenan, who died on Thursday morning of congestive heart failure just three months before his 100th birthday, was a legendary fixture in the department during his decades long tenure that ended in January 1978, police and family said.

“He was called the ‘Gentleman’s cop’,” his daughter Joan Brennan, 66, told The Post Friday in between tears and laughs.

“He was just the most wonderful man. He was a brilliant man, the kind of man that could do the Sunday New York Times crossword puzzle and was interested in everything, any subject, from history to politics.”

He leaves behind his wife of 73 years, Sara Keenan, Brennan, five grandchildren and two great-grandchildren, the family said. John Keenan also had two daughters who passed before him — Mary Ellen who died at 22 in a plane crash and another Sara who died two years ago from cancer.

“My grandfather was an accomplished brilliant man who helped keep New York City and the world safe,” John Keenan’s grandson Kevin Brennan told The Post.

“More importantly, he was exceedingly humble, honest and kind. He was the finest man I’ll ever know and we all miss him dearly.”

Keenan, who counted J.D. Sallinger as a lifelong friend after the two met during World War II in the U.S. Army’s elite Counter Intelligence Corps, started with the NYPD in 1941 before he was drafted to fight the Nazis across the Atlantic, Joan Brennan recounted.

She said despite her father’s many accolades, which also includes the Liberation of Paris, “he never bragged about anything.”

“All he cared about was doing the right thing in his work, that was always his priority. He didn’t do anything for the glory, he just wanted to do the right thing,” Joan Brennan’s daughter and John Keenan’s granddaughter Mary Ellen Brennan, 36, added.

The NYPD’s current Chief of Detectives Dermot Shea took to Twitter to remember John Keenan’s life.

“Truly saddened to learn that former Chief of Detectives John Keenan has passed. A life of epic heroism, he stormed Normandy on D-Day, fought in the Battle of the Bulge & later, as the NYPD Chief of D’s, took the confession from the Son of Sam. We salute you, sir. #FidelisAdMorte,” Shea tweeted.

Keenan often recounted the famous confession he got from David Berkowitz — a serial killer who terrorized the Big Apple during the summer of 1977 in a reign of terror that left six dead and another seven wounded.

When Keenan arrived at Berkowitz’s home and came face to face with the killer for the first time, Berkowitz immediately recognized him.

“I went up to him, & he said, ‘I know you. You’re detective – Chief Keenan.’ And I said, ‘Yeah? Who are YOU?’ He said, ‘I’m the Son of Sam,” Keenan said of the legendary meeting, according to a tweet from NYPD Commissioner James O’Neill.

“The NYPD mourns the passing of retired Chief of @NYPDDetectives John Keenan,” O’Neill added in the tweet.

Joan Brennan said her dad was one of two people Berkowitz sent taunting letters to that promised he’ll “be back!” They were signed “Mr. Monster.”

The daughter said before her dad joined the NYPD, he was a bookworm who worked at the New York Public Library. He grew up in Boro Park, Brooklyn before moving to Rockville Centre, Long Island to raise his family.

At St. Michael’s High School, now known as Xavieran High School, John Keenan graduated as valedictorian. When he finished his time in the army, he was a first lieutenant, Kevin Brennan said.

After the legendary cop and freedom fighter retired from the NYPD in 1978, he went on to become the Vice President of Operations for the New York Racing Association.

He spent the rest of his years in retirement, taking his children and grandchildren on vacations and reading newspapers every day, including The Post.

Joan Brennan said during her dad’s last two days on earth, he was surrounded by his family and his wife didn’t leave his bedside for two days.

Sara Keenan, who was too emotional to speak to The Post, repeatedly thanked him for being so good to her throughout her life, the couple’s daughter said.

“She said you’re so honest and kind and good and you’ve worked so hard and you’ve done everything for us,” Joan Brennan recounted.

“That’s what she repeated all night.”

John Keenan’s wake will be held Sunday at the Macken Mortuary in Rockville Centre between 2 and 4 p.m. and 7 and 9 p.m. His funeral mass will be held on Monday at 11 a.m. at St. Agnes Cathedral.