The 13-year-old was diagnosed with neuroblastoma six years ago, E! Online reported. She recently announced on DeGeneres's show that she had also been diagnosed with pre-leukemia.
"This year I met a very special girl, and today we lost her. Sending my heart to Talia's family," DeGeneres said on Twitter. "I'm so sad."
Talia's Facebook page, "Angels For Talia," was updated Tuesday: "Please lift her beautiful soul, her beautiful light to heaven and please send your love and prayers to her family during this most difficult time,'' the post reads. "God speed little one, may you be free from pain and suffering, may your soul feel the light and love that you brought to so many of us on this Earth during the short time you were here with us. We will miss you more than you will ever know baby girl."
When Talia appeared on her show, DeGeneres arranged for her to become a spokeswoman for CoverGirl.
The makeup company mourned her in a tweet: "Talia Castellano was the meaning of a true CoverGirl."
Talia's YouTube clips giving makeup advice picked up 15 million followers.