Zhang Xu (張旭, fl. 8th century), courtesy name Bogao (伯高), was a Chinese calligrapher of the Tang dynasty.
A native of Suzhou, he became an official during the reign of Emperor Xuanzong. Zhang was known as one of the Eight Immortals of the Wine Cup (飲中八仙), who were a group of Tang dynasty scholars known for their love of alcoholic beverages. Legend has it that whenever he was drunk, he would use his hair as brush to perform his art, and upon his waking up, he would be amazed by the quality of those works but failed to produce them again in his sober state.
Though more well-known for his explosive cursive script, he excelled in the regular script as well. Anecdotes go that he grasped the essence of cursive calligraphy by observing some porters fight for their way with the guard of honor of some princess, and by watching the solo performance of a famous sword-dancer.
He is often paired with the younger Huaisu (怀素) as the two greatest cursive calligraphers of the Tang dynasty, and perhaps even throughout Chinese history. The duo is affectionately referred to as “the crazy Zhang and drunk Su” (顛張醉素).