Plesiosiro is an extinct arachnid genus known exclusively from nine specimens from the Upper Carboniferous of Coseley, Staffordshire, United Kingdom.

[1][2] The genus is monotypic, represented only by the species Plesiosiro madeleyi described by Reginald Innes Pocock in his important 1911 monograph on British Carboniferous arachnids.

[2] Plesiosiro means "close to Siro", which is a genus of cyphophthalmid (Cyphophthalmi); the most primitive group of the living harvestmen (Opiliones).

A 2007 study tentatively recognised a group named Schizotarsata Shultz, 2007[5] comprising Haptopoda, Amblypygi (whip spiders), Uropygi (whip scorpions) and Schizomida (schizomids), which has been recovered subsequently in a number of other phylogenies to include fossils.

A 2014 analysis found that Plesiosiro had chelicerae consisting of two elements, a basal paturon and a fang, which suggests a position within Tetrapulmonata.