Kleshas (Sanskrit: kleśa; Pali: kilesa; Standard Tibetan: nyon mongs,) in Buddhism, are mental states that cloud the mind and manifest in unwholesome actions. Kleshas include states of mind such as anxiety, fear, anger, jealousy, desire, depression, etc. Contemporary translators use a variety of English words to translate the term kleshas, such as: afflictions, defilements, destructive emotions, disturbing emotions, negative emotions, mind poisons, etc.

In the contemporary Mahayana and Theravada Buddhist traditions, the three kleshas of ignorance, attachment, and aversion are identified as the root or source of all other kleshas. These are referred to as the three poisons in the Mahayana tradition, or as the three unwholesome roots in the Theravada tradition.

While the early Buddhist texts of the Pali canon do not specifically enumerate the three root kleshas, over time the three poisons(and the kleshas generally) came to be seen as the very roots of samsaric existence.