Brunei Darussalam, Indonesia, Malaysia, Myanmar, Singapore, Thailand.
Extreme southern Myanmar (southern Tenasserim), south-western and southern Thailand, Peninsular Malaysia, Sumatra (including Nias I), northern Natuna Is and Borneo.
 
Population
Estimated population is unknown (2010).
Plain Sunbird (Anthreptes simplex) [XC34686]
by Iain Woxvold from Danum Valley Field Centre, Danum Valley, Sabah, Borneo, Malaysia (call)
Plain Sunbird (Anthreptes simplex) [XC657096]
by David Farrow from Johor, Panti Forest Reserve, Malaysia (call)
Subspecies
No subspecies.
Birds from Borneo, having different colour gloss on forehead of male, described as subspecies simplicior, possibly worthy of recognition.
Proposed subspecies xanthochlora (from Tenasserim, in southern Myanmar) and euthapsinus (Bunguran, in Natuna Is) considered indistinguishable from those elsewhere in range.