Stylonema alsidii is a species of marine red algae.

The type locality is Trieste, Italy, but it has a worldwide distribution.

The species was first described by Giovanni Zanardini in 1840 as Bangia alsidii.

[2][3] It is one of the algae of the Houtman Abrolhos, found off the coast of Western Australia.

It is susceptible to infection by the parasitic oomycete Pythium porphyrae.