Prosotas dubiosa, the tailless lineblue or small purple lineblue, is a blue butterfly (Lycaenidae) found in Asia to Australia. The species was first described by Georg Semper in 1879.

The butterfly occurs in Sri Lanka and peninsular India and the Himalayas from Sikkim to Assam. The range extends onto Myanmar and Yunnan, and, possibly Thailand, Peninsular Malaysia, Singapore, Hong Kong, Sumatra, Borneo and Java. Possibly Philippines and Sulawesi. Further east the butterfly occurs in Australia, New Guinea and the Solomon islands. In Australia the butterfly occurs from Northern Australia to New South Wales (Coffs Harbour).