"My heart is shattered into a million pieces," the Django Unchained actor wrote in a heartbreaking tribute to his sister.
He went on to write that anyone who knew DeOndra "knew that she was a bright light" and reminisced about how she would steal the show at parties with her dancing, even giving her "boyfriend" Chris Brown a run for his money.
The actor said that while his sister had left a "hole" in his heart, he would fill it with all of the memories that she had given him over the course of her life.
"I love you with every ounce of me... our family is shattered but we will put the pieces back together with your love," Foxx continued, before asking his fans to keep his family in their prayers.
Foxx's celebrity friends soon began sending a flood of supportive comments.
"God Bless you man. My condolences & love to you and your family…" TV presenter Mario Lopez said.
"So sorry for your loss," Black-ish star Yara Shahidi wrote.
Snoop Dogg and Timbaland both commented hearts and prayer hands emojis.
"Here for you brother! No words can ease what you're going through but the Jordan's are here for you and your family!!" Black Panther star Michael B. Jordan wrote.
DeOndra Kept Her Famous Brother "Grounded"
Foxx told NBC's Dateline in 2018 that DeOndra, who was born with the genetic disorder Down Syndrome, kept him "grounded."
"DeOndra has a light on that has been on since she was born," he said.
The Amazing Spider-Man 2 actor said that he remembered her on the day she was born as a "little bundle of joy", and admitted that his family didn't know exactly what Down Syndrome was at the time.
He emphasised that his mother gave DeOndra a normal childhood, encouraging her to join choir and other extracurricular activities.
Foxx also declared that he learned how to live from her.
"You get caught up in all this Hollywood, and it really doesn't matter. She brings you back down to what life is.'
DeOndra Became The Ambassador For A Global Foundation
In her adulthood, DeOndra became an Ambassador for the Global Down Syndrome Foundation.
In a 2011 statement for the Foundation, DeOndra wrote that she knew her family loved her, and that they never set any limits for her.
"I am proud of myself. I have a job at my school. I am the Ambassador for the Global Down Syndrome Foundation and I won the Quincy Jones Exceptional Advocacy Award! I am special," Deondra added.
She enthusiastically participated in the Foundation's Be Beautiful Be Yourself Fashion Show every year, and practised "secret moves" for all of her runways.
Her love of dance was shared with the world by her famous brother. DeOndra made an appearance in Foxx's music video "Blame It," as well as dancing on stage at the Grammy's and some of his concerts.
The Foundation wrote a tribute following her death, calling her an "angel" and describing the collective "shock" and "grief" felt by all who knew her.