Padina japonica is a species of small brown alga found in the tropical and subtropical Indo-Pacific region.

[1] Padina japonica has a flat blade rolled into a circle.

The upper surface is whitish while the underside is brown.

It is similar to Padina australis but rather smaller and the blade is usually unsplit.

[2] Another very similar species is Padina sanctae-crucis; the underside of the latter is more highly calcified, the oogonia and the tetrasporangia are large and globular rather than small and oblong, and there is a single row of tetrasporangial sori rather than the multiple irregular rows of P.