The shield number that slain NYPD Detective Steven McDonald wore when a gunman shot him was retired in a ceremony on Wednesday — after a Queens cop who was wearing the number offered to give it up.

McDonald was shot three times in July 1986 as he questioned a group of robbery suspects in Central Park. After the shooting, he was confined to a wheelchair and needed a ventilator to breathe.

McDonald died more than three decades later as a result of his injuries at the age of 59 on Jan. 10, 2017.

On Wednesday, Mayor Bill de Blasio and Police Commissioner O’Neill presented shield number 15231 to McDonald’s son Conor and wife Patti Ann at a Medal Day ceremony in front of One Police Plaza.

NYPD officer Brendan McNamara, 28, who was wearing the number, agreed for it to be retired.

“For him to give up in honor of my dad means the world to me,” Conor McDonald, now an NYPD sergeant, said after the ceremony as tears formed in his eyes.

“My dad was a good man, and that was very important to him — that shield,” Conor added.

McNamara said he was “shocked” to learn he was wearing the same number as McDonald — but more than happy to give it up when approached by a lieutenant.

“That shield was assigned to me when I graduated the police academy, at random I guess,” McNamara, who serves in 105 Precinct in Queens, said. “As soon as I found out I was shocked.”

McNamara and Conor McDonald had actually met previously when they were both working in Queens, but neither of them knew he was wearing that special shield number. Conor served in the Queens warrant squad in 2016 and spoke to McNamara on several occasions outside the 105th Precinct in Queens Village, he said Wednesday.

“Being a cop I know how important your first shield is,” Conor said. “For him to give up his shield in honor of my father … this is a beautiful moment.”