Abul-Hasan Kūshyār ibn Labbān ibn Bashahri Gilani (971–1029), also known as Kūshyār Gīlānī (Persian: کوشیار گیلانی), was an Iranian mathematician, geographer, and astronomer from Gilan, south of the Caspian Sea, Iran.
His main work was probably done about the beginning of the eleventh century, and seems to have taken an important part in the elaboration of trigonometry. For example, he continued the investigations of Abul Wáfa, and devoted much space to this in his zij (or collection of 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. The tables were translated into Persian 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, extant in Arabic and Hebrew).
He was the teacher of Ahmad Nasawi. He is thought to have died in Baghdad.