Asha (; also arta ; Avestan: 𐬀𐬴𐬀, romanized: aṣ̌a/arta) is a Zoroastrian concept with a complex and highly nuanced range of meaning. It is commonly summarized in accord with its contextual implications of 'truth' and 'right' (or 'righteousness'), 'order' and 'right working'. For other connotations, see meaning below. It is of cardinal importance to Zoroastrian theology and doctrine. In the moral sphere, aṣ̌a/arta represents what has been called "the decisive confessional concept of Zoroastrianism". The opposite of Avestan aṣ̌a is 𐬛𐬭𐬎𐬘 druj, "deceit, falsehood".

Its Old Persian equivalent is arta-. In Middle Iranian languages the term appears as ard-.

The word is also the proper name of the divinity Asha, the Amesha Spenta that is the hypostasis or "genius" of "Truth" or "Righteousness". In the Younger Avesta, this figure is more commonly referred to as Asha Vahishta (Aṣ̌a Vahišta, Arta Vahišta), "Best Truth". The Middle Persian descendant is Ashawahist or Ardwahisht; New Persian Ardibehesht or Ordibehesht. In the Gathas— the oldest texts of Zoroastrianism, thought to have been composed by Zoroaster —it is seldom possible to distinguish between moral principle and the divinity. Later texts consistently use the 'Best' epithet when speaking of the Amesha Spenta, only once in the Gathas is 'best' an adjective of aṣ̌a/arta.