The Ceylon tree nymph or Ceylon tree-nymph (Idea iasonia) is a species of nymphalid butterfly in the Danainae subfamily. It is endemic to Sri Lanka. First described by John Westwood in 1848, the Ceylon tree nymph can be found in both wet and dry zones of Sri Lanka. It is the largest member of the Danaidae family in the country. It is listed as a near threatened species in the IUCN Red List.