Harper revealed on Wednesday (March 6) that she has a rare type of cancer which affects cells in the membrane surrounding the brain.
Moore told People: "I'm absolutely devastated by this news. Valerie has given so much joy, laughter and love to the world.
"I join her fans and send much love and positive thoughts to her and her family during this difficult time."
Harper played Rhoda Morgenstern opposite Moore's Mary Richards in The Mary Tyler Moore Show and its spinoff Rhoda.
The pair also reunited in the TV movie Mary and Rhoda in the year 2000.