Poitier, the first Black man to win an Oscar, was declared the "last of the Great Trees" to fall by Winfrey, who posted a photo of the two together.
"I treasured him. I adored him. He had an enormous soul I will forever cherish. Blessings to Joanna and his world of beautiful daughters," Winfrey wrote.
Oscar winner Viola Davis recalled a dinner with Poitier and described his impact on her life as radical.
"The dignity, normalcy, strength, excellence and sheer electricity you brought to your roles showed us that we, as Black folks, mattered!!!" wrote Davis before she shared some famous words of wisdom Poitier once gave.
"You told us, 'If your dreams do not scare you, they're not big enough'! I put this quote on my daughter's wall."
"I am because of him," actor Colman Domingo wrote on Twitter.
"There is no man in this business who has been more of a North Star for me than Sidney Poitier," actor and film executive Tyler Perry wrote.
Perry shared his memories of a "life changing" trip to South Africa with Poitier alongside the late Cicely Tyson.
"Selfishly, I wanted to hold them both captive for the hours long trip as I literally sat at their feet and listened to their wisdom and experiences," he recalled on Facebook.
Actor Billy Dee Williams described Poitier as a "kind soul who changed the lives of so many, and a hero to all."
"Thank you for kicking down the doors. Thank you will never be enough for your blood sweat tears and determination," Taraji P. Henson wrote.
"We will always hold you in our hearts and forever speak your name," said famed choreographer and actor Debbie Allen.
"Thank you forever," said filmmaker Ava DuVernay.
"He showed us how to reach for the stars," wrote Oscar winner Whoopi Goldberg.
Comedian and actor Kim Fields described Poitier as "Hollywood royalty."
"The doors you opened and paths you created will continue to make way for those with a dream," Kravitz said.
"A great friend, I learned a lot from watching Sidney and how he carried himself with such grace and class," shared Magic Johnson.
"This is a monumental loss," wrote 30 Rock actor Keith Powell, who shared that Poitier had been the inspiration for his career.
"A leader and a groundbreaker," wrote actor Jason Alexander.
"An absolute legend," said Joseph Gordon-Levitt.
A respected activist and humanitarian, Poitier was also remembered in political circles. News of his death was announced in a livestream lead by the prime minister of the Bahamas, Philip Davis.
The national flag is to be flown at half mast in the Bahamas and at its embassies around the world out of respect for Poitier.
He was a "singular talent," said former president Barack Obama, who sent a message of condolences to family and fans.
"What we have lost, the ancestors have gained," wrote Democratic Rep. LaKeshia Myers.