Atronectria thelotrematis, a lichenicolous pyrenomycete growing on Thelotrema lepadinum, is described as new to science from Chile. The species is characterized by black perithecioid ascomata; a brown, not changing color in K and lactic acid, N+ reddish orange exciple resembling textura angularis or textura epidermoidea; 0–4-septate periphyses; a I−, K/I− hymenium with rather rare lipid droplets; the absence of distinct interascal filaments; inoperculate, unitunicate, cylindrical, 8-spored, I−, K/I− asci; and hyaline to occasionally light brown, smooth to mostly finely verruculose, 1-septate, homopolar ascospores, uniseriate in the ascus. A key to ten species of lichenicolous fungi known to occur on Thelotrema is provided. Key words: lichen parasites, South America, taxonomy.