In a short statement, the former President said: "Feeling very sad. Just heard the passing of my good friend Bob Collymore. What a loss!!
"He built Safaricom into the biggest and most prosperous brand in East Africa. He battled his illness with the same passion he approached his work.
"Farewell my friend."
Collymore began receiving treatment for leukemia in 2017, but his condition worsened in recent weeks, Safaricom Chairman Nicholas Nganga, said in a statement.
"He has been undergoing treatment for his condition since then in different hospitals and most recently at Aga Khan University Hospital in Nairobi. In recent weeks, his condition worsened, and he succumbed to the cancer at his home in the early hours of Monday," the company stated.