John Stamos Announces Death of His 'Greatest Friend' Mike Owen

John Stamos is mourning the loss of a dear friend ... announcing the heartbreaking news on social media Monday.

The "Full House" alum posted a poignant tribute to his pal, Mike Owen, who he said he's been friends with since he was just 11 years old.

Stamos said he learned of Mike's passing in a text from his friend's brother ... sharing he was on a flight to Detroit when he received the news.

According to John, Mike died from a "sudden heart attack," though didn't share any other details of the events surrounding the pal's death.

Instead, John penned a lengthy message in honor of his BFF ... remembering Mike as his "everything friend."

He continued ... "From the moment we met, we were inseparable, two boys finding magic in Disneyland and in each other’s company. He was the guy who always let me shine, always happy to be in the shadows, cheering me on. Mike had this way of making the world feel steady. He wasn’t flashy, didn’t need to be the center of attention, didn’t hunger for more than what he had. His gift was something most people spend their lives chasing: contentment."

John recalled their last visit together ... which happened to be 3 weeks ago, when the best friends went to Disney World -- where JS said they were able to be like "kids again."

He added ... "Losing Mike feels like losing a piece of myself, like chapters of my story are missing now. He knew every secret, every joy, every heartbreak."

Signing off his note, John encouraged his followers to appreciate every moment and to tell loved ones how much they mean to you.

John's loss comes almost a week after his "Full House" costar Dave Coulier announced he is battling stage 3 non-Hodgkin lymphoma. John responded to the news by vowing to stay by Dave's side during the health saga ... calling him his "brother from day 1."

Both John and Dave lost their good friend and costar Bob Saget in January 2022 ... the comedian died following an accidental head injury.