An Akathist, akaphist or Acathist Hymn (Greek: Ἀκάθιστος Ὕμνος, "unseated hymn") is a type of hymn usually recited by Eastern Orthodox and Byzantine Catholic Christians, dedicated to a saint, holy event, or one of the persons of the Holy Trinity. The name derives from the fact that during the chanting of the hymn, or sometimes the whole service, the congregation is expected to remain standing in reverence, without sitting down (ἀ-, a-, "without, not" and κάθισις, káthisis, "sitting"), except for the aged or infirm.

The Akathist is also known by the first three words of its prooimion (preamble), Têi hypermáchōi strategôi (Τῇ ὑπερμάχῳ στρατηγῷ, "To you, invincible champion") addressed to Holy Mary (Panagia Theotokos, "The all-holy birth-giver of God").

During Byzantine Catholic and Orthodox Christian religious services in general, sitting, standing, bowing and the making of prostrations are set by an intricate set of rules, as well as individual discretion. Only during readings of the Gospel and the singing of Akathists is standing considered mandatory for all.

The akathist par excellence is the one written for the feast of Annunciation of the Theotokos (25 March). This kontakion was traditionally attributed to Romanos the Melodist since kontakia of Romanos dominated the classical repertoire of 80 kontakia sung during the cathedral rite of the Hagia Sophia, though recent scholarship rejects this authorship like in cases of many other kontakia of the core repertoire. According to the synaxary the origin of the feast is assigned by the Synaxarion to the year 626, when Constantinople, in the reign of Heraclius, was attacked by the Persians and Avars but saved through the intervention of the Most Holy Theotokos. "From that time, therefore, the Church, in memory of so great and so divine a miracle, desired this day to be a feast in honour of the Mother of God ... and called it Acathistus" (Synaxarion). This origin is disputed by Sophocles on the grounds that the hymn could not have been composed in one day, and its twenty-four oikoi contain no allusion to such an event and therefore could not have been composed to commemorate it. However the feast may have originated, the Synaxarion commemorates two other victories, under Leo III the Isaurian, and Constantine Pogonatus, similarly ascribed to the intervention of the Theotokos.

No certain ascription of its authorship can be made. It has been attributed to Patriarch Sergius I of Constantinople, whose pious activities the Synaxarion commemorates in great detail. J.M. Quercius (1777) assigns it to George Pisida, deacon, archivist, and sacristan of Hagia Sophia whose poems find an echo both in style and in theme in the Akathist; the elegance, antithetic and balanced style, the vividness of the narrative, the flowers of poetic imagery being all very suggestive of his work. His position as sacristan would naturally suggest such a tribute to the Theotokos, as the hymn only gives more elaborately the sentiments condensed into two epigrams of Pisida found in her church at Blachernae. Quercius also argues that words, phrases, and sentences of the hymn are to be found in the poetry of Pisida. Leclercq finds nothing absolutely demonstrative in such a comparison and offers a suggestion which may possibly help to a solution of the problem.

Before the turn of the 21st century the Akathist was usually assigned to the 6th or 7th century but more recent scholarship, driven by the work of Leena Peltomaa, has argued for a 5th century origin on the basis of theological content. Cunningham concurs with Peltomaa's analysis of the hymn's Christology but postulates, from its "highly developed poetic form" and elaborate invocations of Mary, a somewhat later provenance of the latter 5th/early 6th century. In contrast, Shoemaker reasons that "Peltomaa's [arguments] for dating the Akathist hymn to the period before Chalcedon would seem to apply equally if not even more so" for a collection of Marian hymns within the Georgian Chantbook of Jerusalem that he contends are of primarily pre-Chalcedonian authorship. Following Renoux he argues that the hymns, which are colourful and invocatory, contain theology closest to mid-5th century homilists, bearing witness to a highly developed cult of Mary at an earlier period than previous generations of scholars had appreciated. Reynolds summarises prevailing opinion as being in favour of a date for the Akathist "somewhere between the Councils of Epheus and Chalcedon." Similarly, Arentzen observes that most scholars now favour an early provenance.

Since the 14th century the Akathist moved from the menaion to the moveable cycle of the triodion, and the custom established that the whole hymn was sung in four sections throughout Lent. As such it became part of the service of the Salutations to the Theotokos (used in the Byzantine tradition during Great Lent).

Apart from its usual dedication to the menaion and the early custom to celebrate kontakia during the Pannychis (festive night vigil celebrated at the Blachernae chapel), the Akathist had also the political function to celebrate military victories or to ask during wars for divine protection intermediated by prayers of the Theotokos. This function is reflected within the synaxarion.