Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified, have an oxic, highly productive, thin surface layer, and have anoxic, sulfidic bottom water.

The word euxinia is derived from the Greek name for the Black Sea- Εὔξεινος Πόντος (Euxeinos Pontos)- which translates to "hospitable sea". Euxinic deep water is a key component of the Canfield ocean, a model of oceans during the Proterozoic known as the Boring Billion proposed by Donald Canfield, an American geologist, in 1998.

There is still debate within the scientific community on both the duration and frequency of euxinic conditions in the ancient oceans. Euxinia is relatively rare in modern bodies of water, but does still happen in places like the Black Sea and certain fjords.