The Ceylon tree nymph (Idea iasonia) is a species of nymphalid butterfly in the subfamily Danainae.

First described by John Westwood in 1848, the Ceylon tree nymph can be found in both wet and dry zones of Sri Lanka.

For several years it was considered to be only an island race of I. lynceus, a species found in eastern Asia.

[2] The Ceylon tree nymph has a wingspan of 110–155 mm, making it the largest member of the family Danaidae in Sri Lanka.

[3] The surface area of the Ceylon tree nymph's wings is relatively large when compared with its weight; this allows it to fly with little effort and stay aloft for long periods of time.

The smaller and darker variety is found in the wet zone of Sri Lanka from sea level to about 5,000 feet (1,500 m).

One account by naturalists Lionel de Nicéville and N. Manders dating from about 1900 describes the larva as "velvety black with four pairs of long filamentous tentacles" with each segment of its body marked with a pale yellow band.