紫萁目是蕨类植物门里的一目，只包括紫萁科一科。紫萁目的叶子通常呈现丛生状状，羽毛状的排列，繁衍用的孢子密生于叶脊。紫萁目比其他大多数的蕨类要来得大。一般来说，该种植物都生长在湖沼湿地。

紫萁目里的蕨类会形成厚重的根茎，并带有茂密的坚靭根部。许多物种会形成短树干，但只有高约8公分至10公分而已；如在「块茎蕨属」中，即有时会因为其树干被认为是树蕨，虽然它们相对较短。

其叶组织由如假紫萁非常粗糙、几乎像是皮革般，到如薄膜蕨属易脆且透明的都有。

The Osmundaceae (Royal Fern Family) is a family of four genera and 15-25 species. It is the only fern family of the order Osmundales an order in the class Polypodiopsida (Filicopsida, Pteridopsida, or Leptosporangiate ferns) or in some classifications the only order in the class Osmundopsida. This is an ancient (known from the Upper Permian) and fairly isolated group that is often known as the "flowering ferns" because of the striking aspect of the ripe sporangia in Osmunda and Osmundastrum. In these genera the sporangia are borne naked on non-laminar pinnules, while Todea and Leptopteris bear sporangia naked on laminar pinnules. Ferns in this family are larger than most other ferns.