Mark Ruffalo has shared a heartfelt letter he once wrote to Robert Redford but never sent.

On Tuesday, a representative for the Ordinary People director announced that he had died in his sleep at his home in Sundance, Utah. Redford was 89.

Following the sad news, Ruffalo took to Instagram to post a lengthy letter he penned to his "hero" after recently re-watching his 1976 film, All the President's Men.

"You have had the most profound impact on my life and career in so many ways," he began." You have been a hero of mine."

Ruffalo went on to note that he was "so proud" that he had the opportunity to act opposite Redford in the 2001 prison drama, The Last Castle.

"I told you before. The short time together we had on The Last Castle was seminal to me. To be acting opposite of you. To have you be so generous with me. I felt so safe with you and looked after," the 57-year-old continued.

Elsewhere in the note, the Spotlight star thanked Redford for his contribution to cinema and activism.

"I am proud that I got to spend some time with you and proud I have been able to follow in your footsteps even in the smallest of ways. You are still a hero to me. Thank you for being my teacher and role model," he added.

In the accompanying caption, Ruffalo explained that he wrote the letter when he learned Redford was "ill" but decided not to send it to his "American hero".

Other celebrities to pay tribute to the Hollywood icon via Instagram include Michelle Pfeiffer, who posted a throwback of herself and Redford on the set of 1996 drama Up Close & Personal, and Samuel L. Jackson, who shared a still of himself and the actor from a scene in 2014's Captain America: The Winter Soldier.

"It was a dream come true to work on screen with Mr Redford. Sometimes your heroes don't disappoint & truly are as Great as you imagined," Jackson praised.

Redford is survived by his wife Sibylle Szaggars, two daughters, and seven grandchildren.