Mírzá Mihdí (Persian: ميرزا مهدي 1848 – June 23, 1870) was the youngest child of Bahá'í founder Bahá’u’lláh and wife Navváb. He was given the title Ghusn-i-Athar ("Purest Branch").
He was born in Tehran, Persia as Mihdí Núrí.
Mírzá Mihdí was his mother's favourite child. Mihdí did not remember his once luxurious life as at the age of four his father was arrested and imprisoned on account of his faith. The family were left in penury and his sister later recollected how the siblings would hold each other and weep. When his family were exiled to Baghdad, the family thought it necessary to leave Mírzá Mihdí in Persia as a result of his health. He was left to the care of his maternal great-grandmother and his paternal aunt. The separation was hard for his family, especially his mother.
At the age of twelve, he rejoined his family in Baghdad. There he accompanied his father through all his exiles and grew very close to him. He later became the amanuenses of Bahá’u’lláh and a staunch companion. He was beloved by the Bahá’ís; revered in a similar sense to ‘Abdu’l-Bahá and was noted for his meekness, piety, purity and sincerity. Whilst rapped in prayer, chanting the Ode of the Dove (a prayer revealed by his father during the period in Kurdistan) and pacing on the rooftop, Mírzá Mihdí fell through a skylight, and the wooden crate that he fell onto pierced his ribs and injured his lungs. Bahá’u’lláh was horrified and asked for a physician to attend his son. However, by this time it was too late. His mother was especially hurt and poignantly caressed her blood-soaked son shortly before his death. Bahá’u’lláh was heard lamenting “Mihdí! O Mihdí!” as his son was dying. His companions surrounding him on his death-bed remember his courtesy despite his pain, and even apologizing that he was lying before their presence.