The Essenes (; Hebrew: אִסִּיִים‎, Isiyim; Greek: Ἐσσηνοί, Ἐσσαῖοι, or Ὀσσαῖοι, Essenoi, Essaioi, Ossaioi) were a mystic Jewish sect during the Second Temple period that flourished from the 2nd century BCE to the 1st century CE.

The Jewish historian Josephus records that Essenes existed in large numbers; thousands lived throughout Roman Judaea. They were fewer in number than the Pharisees and the Sadducees, the other two major sects at the time. The Essenes lived in various cities but congregated in communal life dedicated to voluntary poverty, daily immersion, and asceticism (their priestly class practiced celibacy). Most scholars claim they seceded from the Zadokite priests.

The Essenes have gained fame in modern times as a result of the discovery of an extensive group of religious documents known as the Dead Sea Scrolls, which are commonly believed to be the Essenes' library. These documents preserve multiple copies of parts of the Hebrew Bible untouched from possibly as early as 300 BCE until their discovery in 1946.

The first reference to the sect is by the Roman writer Pliny the Elder (died 79 CE) in his Natural History. Pliny relates in a few lines that the Essenes possess no money, had existed for thousands of generations, and that their priestly class ("contemplatives") do not marry. Unlike Philo, who did not mention any particular geographical location of the Essenes other than the whole land of Israel, Pliny places them somewhere above Ein Gedi, next to the Dead Sea.

Josephus later gave a detailed account of the Essenes in The Jewish War (c. 75 CE), with a shorter description in Antiquities of the Jews (c. 94 CE) and The Life of Flavius Josephus (c. 97 CE). Claiming firsthand knowledge, he lists the Essenoi as one of the three sects of Jewish philosophy alongside the Pharisees and the Sadducees. He relates the same information concerning piety, celibacy, the absence of personal property and of money, the belief in communality, and commitment to a strict observance of Sabbath. He further adds that the Essenes ritually immersed in water every morning – a practice similar to the use of the mikveh for daily immersion found among some contemporary Hasidim –, ate together after prayer, devoted themselves to charity and benevolence, forbade the expression of anger, studied the books of the elders, preserved secrets, and were very mindful of the names of the angels kept in their sacred writings.

Pliny, also a geographer, located them in the desert near the northwestern shore of the Dead Sea, where the Dead Sea Scrolls were discovered.

There were multiple minor subsects of the Essenes, including the Hemerobaptists, Bana'im and disputedly the Maghāriya.