Ayelet Galena, 1, suffers from a rare congenital disorder and requires a bone marrow transplant in order to survive.
50 Cent followed Rihanna in getting a blood test to see if they could be potential donors for the baby girl.
Rihanna later tweeted to her followers to offer their help, saying: "Can you help me find a lifesaver for baby Ayelet and other patients? Get swabbed (tested) because maybe you can save a life!"
Leighton Meester, Linkin Park and Pharrell Williams have also lent their support by highlighting Galena's illness.
Rihanna had previously lent her support to six-year-old leukaemia sufferer Jasmina Anema, who passed away in January.