The retired-Marine husband of a White Plains police officer used his wife’s service weapon to commit suicide in their Upper East Side apartment Sunday afternoon, police sources said.

Sheamus Cullen, 50, was found by his wife, Jeanette Cullen, a police officer, with a gunshot wound under his chin in the bedroom of their Park Avenue home around 4 p.m., sources said.

Jeanette had just returned from a swimming class with the couple’s baby when she made the grim discovery, police said.

Her 9mm service weapon was by her husband’s side, the sources said.

Jeanette frantically called 911 for help, and first responders rushed Sheamus to Mt. Sinai Hospital, but he couldn’t be saved.

Sheamus served as a Marine Corps infantry squad leader during Operation Desert Storm, according to a social media page.

He had grappled with depression and post-traumatic stress disorder stemming from his service, according to a police source.

He had also sent text messages to his friends and wife expressing suicidal thoughts, the source added.

“He was a good man and a good friend,” grieving pal Sevdet Kukaj told The Post over the phone. “He served our country.”