Mírzá Músá (Persian: میرزا موسى; surnamed Áqáy-i-Kalím; died 1887) was the only full brother of Bahá'u'lláh, meaning that they shared the same mother and father. He was later named by Shoghi Effendi as one of the nineteen Apostles of Bahá'u'lláh.
The life of Mírzá Músá was so inextricably bound up with that of Bahá'u'lláh himself, that his life and background mirror the life and travels of Bahá'u'lláh. He was an integral part of correspondence between Bahá'u'lláh and the Bahá'ís. He experienced the same imprisonment, exile, assaults, and degrading circumstances that were given to the small band of family members associated with Bahá'u'lláh and `Abdu'l-Bahá. In the history of the Bahá'í cause, Mírzá Músá stands out as a loyal and faithful follower until the end.
Bahá'u'lláh used Mírzá Músá as an example to show his respect for the law. When an official expressed hesitation to inflict punishment on one of the followers of Bahá'u'lláh who had committed a crime, he replied:
Mírzá Músá's son, Mirza Majdi'd-Din for a time transcribed the Tablets of Bahá'u'lláh, but later became "the most redoubtable adversary of `Abdu'l-Bahá" by supporting Mírzá Muhammad `Alí, the arch-breaker of the Covenant. He was the one who read the Kitáb-i-'Ahd in front of the family upon the passing of Bahá'u'lláh.