Russula firmula is a species of mushroom found in Europe and parts of Asia that has dark red-purple to red-brown cap.

It is one of about 750 species in the genus Russula, which is distributed worldwide.

[1] Russula firmula has a yellow spore print.

There are incrustations on its pileocystidia which stain bright pink after about 30 minutes in sulphovanillin, in contrast with the rest of the pileocystidia which stain black.

The terminal cells at the cap margins are cylindrical, though tapered narrowly at one end.