Cooper was attending Elliott's hand and footprint ceremony.
The 44-year-old director attended Elliott's hand and footprint ceremony at TCL Chinese Theatre in Hollywood, California, on Monday and got emotional while introducing the 74-year-old. Elliott starred in A Star Is Born as Bobby, the brother to Cooper's character, Jackson Maine.
"Sam is one of the most talented, generous, humble actors I've ever had the pleasure to work with. And acting alongside him was a dream fulfilled for me," Cooper gushed. "He is the kind of guy who raises the bar for everyone, whether you're in front of the camera or behind the camera. Directing Sam was and will remain one of the highest points of my career."
"Personally, I owe a lot to Sam for the success of this film. Not only because of his brilliant performance, but because, if he hadn't agreed to it, honestly, I don't know what I would've done," Cooper admitted with a laugh. "Thank you, Sam, for saying yes, for trusting me. Thank you for letting me steal your voice."
Cooper went on to emotionally compare his real-life relationship with Elliott to their characters' onscreen one.
"You know, in the movie, Jack may struggle with telling his brother how he feels about him, I've got no problem telling you, Sam, just how much I love and admire you. You are my brother," Cooper said, getting choked up. "You've received many honors in your career, I know you will receive many more, but this one, my friend, is written in stone."
When it was his turn to speak, Elliott joked about how his early career as a day laborer, when he "had my feet and hands in cement a good part of the time," prepared him for this honor, before turning his attention to Cooper.
"Bradley... if you look [at] what's behind me here I'd say it's safe to say that the reason I'm here is because of your film," Elliott tearfully noted of the A Star Is Born posters behind him. "And I love you too, pal."
Also in attendance at the special event was Elliott's co-star Lady Gaga. The 32-year-old actress got emotional at the ceremony too, and was even spotted with tears in her eyes.
The "Shallow" singer also gave the honoree a hug and accepted a kiss on the cheek from him too.
On her Instagram Story, Gaga said she was "so excited" to be at the event and expressed her love for Elliott.
The three Star Is Born actors also posed for pics together, even embracing in an incredibly sweet group hug.
Gaga and Cooper weren't the only celebs to turn out to support Elliott! Ashton Kutcher, his co-star on The Ranch, was also in attendance.
Gaga and Cooper's attendance marks their first public appearance since "Shallow," the acclaimed song from A Star Is Born, took home the Golden Globe for Best Original Song. Watch the video below to learn more about the track: