Posnjakite is a hydrated copper sulfate mineral.

It was discovered in the Tungsten deposit of Nura-Taldy[4] in Karaganda Region in Kazakhstan and described in 1967 by Aleksandr Ivanovich Komkov (1926–1987) and Yevgenii Ivanovich Nefedov (1910–1976)[5] and named after geochemist Eugene Valdemar Posnjak (1888–1949).

[6] Posnjakite is an uncommon but widespread secondary mineral in the oxidised zone of copper sulfide deposits, which may be of post-mine formation.

It is associated with brochantite, langite, devilline, serpierite, woodwardite, wroewolfeite, aurichalcite, azurite, malachite and chalcopyrite.

[4]