Hoffman, 46, who won an Oscar for his role in blockbuster 'Capote', was found dead in his New York City apartment of an apparent drug overdose on February 2.
Hoffman, considered one of the finest actors of his generation, is best known for his performances in 'The Scent of a Woman', 'The Master', 'Moneyball' and 'The Hunger Games: Catching Fire' among others.
Hanks, who starred with him in Charlie Wilson's War', said it was a horrible day for those who worked with Hoffman.
Clooney, who worked with Hoffman in 'The Ides of March', said: "There are no words. It's just terrible."
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Veteran actor De Niro said this should not have happened.
"I'm very, very saddened by the passing of Phil. He was a wonderful actor. This is one of those times where you say 'this just shouldn't be. He was so young and gifted and had so much going, so much to live for'. My family and I send our deepest condolences to his family," De Niro said.
Oscar-winner Jeff Bridges said the industry has lost a treasure after the death of his 'The Big Lebowski' co-star.
"I'm so shocked, and so sad hearing of Philip Seymour Hoffman's death. I enjoyed playing with him on 'The Big Lebowski'. He was such a wonderful guy, and so damn talented, a real treasure. My thoughts and prayers are with his family."
Hoffman, who played the role of chief Gamesmaker Plutarch Heavensbee in the final two 'Hunger Games' films, was mourned by the cast.
"Words cannot convey the devastating loss we are all feeling right now," said a statement from Francis Lawrence, Suzanne Collins, Nina Jacobson, Jon Kilik and Jennifer Lawrence on behalf of the 'Hunger Games' cast and crew.