Rex begonia (Begonia rex-cultorum) is a semi-tropical perennial plant normally grown as an outdoor container plant or houseplant. The forms sold commercially are carefully cultivated varieties of the B. rex species native to parts of eastern Asia.

Use a porous potting mix in a relatively shallow pot, and feed the plant regularly. Keep the plant moist, but avoid overwatering. Be especially careful of directly spraying the leaves, as standing water will encourage powdery mildew, which is a disaster on a plant that is cultivated for its beautiful foliage.

Rex begonias like bright, indirect light year-round. Because they are not aggressive bloomers, they can tolerate less light than other begonias and will thrive under fluorescent lights.

Begonia rex, the king begonia, is a species of flowering plant in the family Begoniaceae. It is found from Arunachal Pradesh to southeast China, and has been introduced to Bangladesh, Cuba, and Hispaniola.

It is a parent to over 500 cultivars in the Begonia Rex Cultorum Group of houseplants.  Other parents in the multitude of crosses made during the creation of the Group include Begonia annulata, B. cathayana, B. decora, B. diadema, B. dregei, B. grandis, B. hatacoa, B. palmata, and B. xanthina.