Asplenium vespertinum is a species of fern known by the common name western spleenwort.

A global phylogeny of Asplenium published in 2020 divided the genus into eleven clades,[1] which were given informal names pending further taxonomic study.

A. vespertinum belongs to the "Diellia subclade" of the "A. trichomanes clade".

Members of the clade grow on rocks and have once-pinnate leaf blades with slender, chestnut- to dark-brown stalks.

The Hawaiian species presumably represent an evolutionary radiation following colonization from California.