The singer put out a plea to help find marrow for 41-year-old New York lawyer Lisa Gershowitz-Flynn.
"Today a 32-year-old guardian angel consented to be my donor, even though in all likelihood she has never met me and does not know who I am," Flynn wrote in an email to supporters.
"While we cannot verify the source of the donor, this angel was most likely found through the efforts of my family and friends, as well as complete strangers who heard about my story and took it upon themselves to aid in the effort to find a donor."
After Rihanna started working with DKMS to find help for Flynn, the donor centre was contacted by more than 5,000 readers.
Katharina Harf, executive vice president of DKMS, said: "We actually recruited and registered roughly 2,000 new donors, just from phone calls and emails we got – and it's continuing."
Flynn was able to find a match and is now awaiting the transplant.
"My life will be saved because of you, my family and friends (old and new). My children (Michael, 5, and Alexandra, 3) will have their mother, my husband will have his wife, and my parents will have their daughter," said Flynn.