Abul-Hasan Kūshyār ibn Labbān ibn Bashahri Daylami (971–1029), also known as Kūshyār Daylami (Persian: کوشیار دیلمی), was an Iranian^[1][2] mathematician, geographer, and astronomer from Daylam, south of the Caspian Sea, Iran.

Kūshyār Daylami's main work was probably done about the beginning of the 11th century, and seems to have taken an important part in the elaboration of trigonometry. He continued the investigations of the 10th century mathematician and astronomer Abul Wáfa, and devoted much space to this in his zīj (book of astronomical tables) az-Zīj al-Jamī wal-Baligh ("The Comprehensive and Mature Tables"), which incorporated the improved values of the planetary apogees observed by al-Battani.^[3] The tables were translated into the Persian language before the end of the century. He wrote also an astrological introduction and an arithmetic treatise Kitab fi usul hisab al-hind (Principles of Hindu Reckoning, that is extant in both Arabic and Hebrew).

Kūshyār Daylami's pupils included the Persian mathematician Al-Nasawi. Kūshyār Daylami is thought to have died in Baghdad.