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) [XC424694]
by Greg Irving from Kubah National Park, Frog Pond area, Sarawak, Malaysia (begging call)
Plain Sunbird (Anthreptes simplex) [XC114427]
by Mike Nelson from Johor, Panti Forest Reserve, Bunker Trail, c.1km, 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.