紫萁（学名：Osmunda japonica）为紫萁科紫萁属下的一个种。

Osmunda japonica (Japanese royal fern or Japanese flowering fern; syn. Osmunda nipponica Makino) is a fern in the genus Osmunda native to east Asia, including Japan, China, Korea, ******, and the far east of Russia on Sakhalin.

It is a deciduous herbaceous plant which produces separate fertile and sterile fronds. The sterile fronds are spreading, up to 80-100 cm tall, bipinnate, with pinnae 20-30 cm long and pinnules 4-6 cm long and 1.5-2 cm broad; the fertile fronds are erect and shorter, 20-50 cm tall.