Grammitis diminuta is a fern in the family Polypodiaceae.

The Latin specific epithet diminuta means "decreased" or "diminished", with reference to the tapered frond base.

It has a stout, erect rhizome with light brown, lanceolate scales.

Its simple fronds combine a short stipe with a narrowly elliptic lamina 3–15 cm long and 0.4–0.8 cm wide.

[1] The fern is endemic to Australia’s subtropical Lord Howe Island in the Tasman Sea; it is confined to the cloud forest on the upper slopes and summits of Mounts Lidgbird and Gower.