The 'Umbrella' singer, who received a letter from 26-year-old Nigerian sledder Seun Adebiyi last year, has promised to raise awareness of his condition and help him find a bone marrow donor.
Rihanna said: "Leukaemia interrupted Seun's dream of being the first Nigerian in the Winter Olympics, but together we can help him make his dream come true. We need to find him a bone marrow donor!"
Grey's Anatomy star Justin Chambers has also vowed to help Adebiyi through non-profit bone marrow donor centre DKMS.
The athlete told Tonic: "I am deeply touched by their plea for donors, not just for me, but for others as well. Their voices are so powerful and it encourages me how they used their influence to raise awareness about this important issue. That was incredible."