Arachnothryx is a genus of flowering plants in the family Rubiaceae.

[1] Arachnothryx was named by Jules Émile Planchon in 1849.

[2][3] This generic name is derived from Ancient Greek: arachne or arachnos, "a spider", and thrix or trichos, "hair".

[4] As currently circumscribed, Arachnothryx is paraphyletic over Cuatrecasasiodendron, Gonzalagunia, and Javorkaea.

Some authors have included these genera in a broadly defined Rondeletia, but molecular phylogenetic studies have shown that they are more closely related to Guettarda than to Rondeletia.