John Stamos Mourns Best Friend of 50 Years After His 'Sudden' Death: 'Feels Like Losing a Piece of Myself'

John Stamos is saying goodbye to his best friend.

The Full House alum's longtime pal and "brother," Mike Owen, recently died of a "sudden heart attack," Stamos announced in a Nov. 18 Instagram post .

The star shared the news alongside photos of the duo throughout their 50 years of friendship, as well as a heartfelt message, beginning with how he first learned of Owen's sudden death.

The actor, 61, said he was on a flight to Detroit when he got a text from Owen's brother that read, "Call me," a message that made his stomach drop, he said. "'Is Mike OK?' I sent back," Stamos wrote. "The response took forever, those cruel three dots hovering on my screen. Finally, just two letters: 'No.' "

"Mike Owen, my best friend since I was 11, was gone," he wrote. "A sudden heart attack. Just like that. The world I'd built around our friendship shifted, surreal and devastating."

But to Stamos, Owen wasn't just a friend. "He was the friend," the actor wrote. "My everything friend. 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," he continued. "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."

Owen "carried our shared history with a reverence I didn't always understand," Stamos wrote. "Now, I think he knew something I didn't: how fleeting life is. He held onto our memories not to live in the past, but to savor the beauty of our bond."

The best friends spent some of their final moments together just three weeks ago, the actor shared, describing the time as "one last gift" from Owen.

"I flew him to Disney World, and we spent days like we were kids again—laughing, reminiscing, doing old bits, sitting on familiar benches, soaking in the magic of our friendship," Stamos recalled. "It was pure joy, and I'll treasure it forever."

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

But, Stamos explained, he has a plan for continuing Owen's legacy and doing "what he'd want."

His lifelong friend would want him "to hold onto the love and laughter we shared and to remind you—don't wait," he wrote. "Say the thing. Book the flight. Tell the people you love how much they mean to you, today!"

Finishing the post with a final goodbye to Owen, Stamos said, "Mike, I'll miss you every day."