The Elcesaites, Elkasaites, Elkesaites or Elchasaites were an ancient Jewish Christian sect in Lower Mesopotamia, then the province of Asoristan in the Sasanian Empire that was active between 100 and 400 CE. The members of this sect, which originated in the Transjordan, performed frequent baptisms for purification and had a Gnostic orientation.

The movement blended elements of Second Temple Judaism, early Jewish Christianity, Gnosticism, and apocalyptic mysticism, and it is primarily known through the writings of early Church Fathers such as Hippolytus of Rome, Origen, and Epiphanius of Salamis.

The name of the sect derives from the alleged founder, Elkhasaí (Koine Greek: Ἠλχασαΐ in Hippolytus), Elksai (Ἠλξαί in Epiphanius), or Elkesai (Ελκεσαΐ in Eusebius, and Theodoret).

The sect is directly mentioned only in the commentaries on "heresies" by Early Church Fathers.

Hippolytus of Rome (Refutation of All Heresies, IX, 8–13) records that in the time of Pope Callixtus I (217–222 AD), a Jewish Christian called Alcibiades of Apamea came to Rome, bringing a book which he said had been received in Parthia by a just man named Elchasai. According to Alcibiades, the book had been revealed by an angel 96 miles (154 km) high (337,920 cubits), 16 miles (26 km) broad (56,230 cubits) and 24 miles (39 km) (84,480 cubits) across the shoulders, whose footprints were 14 miles (23 km) long, 6 miles (9.7 km) wide and 2 miles (3.2 km) deep. This giant angel was the Son of God, who was accompanied by his sister, the Holy Ghost, of the same dimensions. Alcibiades announced that a new remission of sins had been proclaimed in the third year of Trajan (100 AD), and he described a baptism which should impart this forgiveness even to the grossest sinners.

Hippolytus' commentary starts in book 9, chapter 8. In his next section, Hippolytus recounts that Alcibiades teaches the natural birth, preexistence and reincarnation of Jesus, which Louis Ginzberg suggested in 1906 may relate to the concept of Adam Kadmon, and also that Alcibiades teaches circumcision and the Law of Moses. Hippolytus then goes on at length to describe the group's teaching on baptism. For all sins of impurity, even against nature, a second baptism is enjoined "in the name of the great and most high God and in the name of His Son the great King", with an adjuration of the seven witnesses written in the book (sky, water, the holy spirits, the angels of prayer, oil, salt and earth). One who has been bitten by a mad dog is to run to the nearest water and jump in with all his clothes on, using the foregoing formula, and promising the seven witnesses that he will abstain from sin. The same treatment – forty days consecutively of baptism in cold water – is recommended for consumption and for the possessed. In his chapter 11, Hippolytus discusses in more detail the teaching of the book including Elchasai's Sabbatarian teaching and the instruction not to baptise under certain astrological stars. Hippolytus concludes his review of the Elcesaites in Refutations, book 10, chapter 12 with a general exhortation to avoid heresy which gives away no more information.

Adolf von Harnack (1898) reads "was proclaimed" instead of "has been proclaimed" (as if eúaggelisthênai and not eúeggelísthai), and thus inferred that a special year of remission is spoken of as past once for all – and that Alcibiades had no reason for inventing this, so that Adolf Bernhard Christoph Hilgenfeld (1884) was right in holding that Elchasai really lived under Trajan, as Epiphanius of Salamis supposed.

Eusebius (History 6.38) records a summary of a sermon on Psalm 82 delivered in Caesarea by Origen c. 240–250 AD which warns his audience against the doctrine of "the Elkesaites". Eusebius' record of this sermon forms the second source on the group. According to Eusebius, Origen regarded the heresy as quite new, and states that the group deny the writings of Paul, but claim to have received a new book from heaven.