John Stamos Suffers Devastating Loss: 'Feels Like Losing a Piece of Myself'

John Stamos is mourning a major loss, as the longtime actor recently revealed his dear pal passed away.

The Full House star took to Instagram to share the heartbreaking news, posting a sweet tribute to his childhood best friend, Mike Owen, on Monday, Nov. 18.

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While sharing their history together with a collection of vintage throwback photos as well as more recent ones, he explained how he learned of the death of his "greatest friend."

"On a flight to Detroit to visit my friend Dave, I got a text from my best friend’s brother: 'Call me.' My stomach dropped. 'Is Mike OK?' I sent back. The response took forever, those cruel three dots hovering on my screen. Finally, just two letters: 'No,'" he opened the lengthy caption of his saddening social media upload.

Stamos explained that Owen has been his best friend since the young age of 11, and that he died of "a sudden heart attack. Just like that," causing a "surreal and devastating" blow.

"Mike wasn’t just my friend. He was the friend. My everything friend. From the moment we met, we were inseparable, two boys finding magic in Disneyland and in each other’s company," the Fuller House actor wrote.

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He continued: "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. It wasn’t complacency. It was peace."

"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," the actor added.

Stamos' major loss comes just days after he offered support for his former costar and "brother from day 1," Dave Coulier, who announced his cancer diagnosis. "I’ll be by your side through it all," the Grandfathered actor wrote on Instagram alongside several photos of the pair from over the years.

Related: Dave Coulier Shares Exciting Family Update After Revealing Cancer Diagnosis

Wrapping up his tribute to his deceased buddy, the 61-year-old closed, "Mike, I’ll miss you every day. You were my brother, my compass, my greatest friend. You taught me to find joy in simplicity, to laugh through life’s hardest moments, and to treasure what we have while we have it. I love you. Always. Rest easy, my friend."

Followers and friends poured into the comment section with condolences for the star's loss.

"I’m so sorry ❤️‍🩹. Sending all the love to you," Candace Cameron Bure penned, while another comment read, "😢 heartbroken for you, sending my deepest condolences."

A third sent virtual "hugs" his way amid a trying time: "This year has been hard."

Next: John Stamos Sends a Rare Political Message Ahead of Election Day