Papilio prexaspes, the blue Helen, is a swallowtail butterfly found in Southeast Asia.

The taxonomic description below is of race prexaspes and is taken from Charles Thomas Bingham's 1907 book (in the public domain): Closely resembles Papilio chaon, from which it differs as follows: smaller; fore wing more produced, its termen concave.

Fore wing with an ill-defined broad pale discal band perceptibly widened and becoming whitish opposite apex of cell.

Underside, fore wing: the transverse discal hand white and much more prominent than on the upperside.

Hind wing: the postdiscal series of blue lunules generally complete and well-marked: the rest as in the male.