This species is native to Peninsular Malaysia, where it is known only from Sungai Mangga, Upper Perak.  It grows on steep earth banks above the river.  From Begonias of Peninsular Malaysia by Ruth Kiew,

“Begonia koksunii is one of the most beautiful Malaysian begonias with its silver variegated leaves and rosette habit.  It is named in houour of its collector Mr. Yap Kok-Sun….it falls within section Platycentrum in possessing a rhizomatous habit, ovaries and fruits with two locules each with two placentas, and fruits with one wing much longer than the other two.  It is a very local species known only from one place.  Logging in the area is currently posing a threat to the long-term survival of this species.”