Cortinarius subsaniosus is a species of webcap mushroom known from north and central Europe, where it grows on sandy soil in association with willows.

Along with five other British webcaps, C. subsaniosus was selected by Kew Gardens as a highlight of taxa described by the organisation's staff and affiliates in 2020.

Cortinarius subsaniosus was described in a 2020 research note in the journal Fungal Diversity by Kare Liimatainen and Tuula Niskanen.

The description was based on a collection made by S. E. Evans in 1999 at Sandscale Haws, a nature reserve near Barrow-in-Furness, Cumbria, England.

In a year-end round-up, Kew scientists selected ten highlights, one of which was six newly described British Cortinarius species: C. subsaniosus described from Cumbria; C. britannicus from Caithness; C. scoticus and C. aurae from the Black Wood of Rannoch; C. ainsworthii from Brighton; and C. heatherae from Heathrow Airport.

The spores are moderately dextrinoid, meaning that they stain a reddish brown when tested with Melzer's reagent or Lugol's solution.