Zhang Guo, better known as Zhang Guolao, is a Chinese mythological figure and one of the Eight Immortals in the Taoist pantheon. Among the Eight Immortals, Zhang Guolao, Zhongli Quan and Lü Yan were real historical figures. His existence is said to have begun around the middle or end of the 7th century, and ended approximately in the middle of the 8th. The epithet "Lao" added at the end of his name means "old".

Zhang was a Taoist fangshi (translated as "occultist-alchemist") who lived as a hermit on Zhongtiao Mountain (中條山; southeast of present-day Yongji, Shanxi) in Hengzhou (恆州) during the Tang dynasty. By the time Wu Zetian came to power, he claimed to be several hundred years old. A strong believer in the magic of necromancy, he also declared that he was a Grand Minister to the mythical Emperor Yao in his previous life. Zhang also had a love for wine and winemaking. He was known to make liquor from herbs and shrubs as a hobby. Other members of the Eight Immortals drank his wine, which they believed to have healing or medicinal properties. He was also known to be a master of qigong and could go without food for days, surviving on only a few sips of wine.

Zhang was the most eccentric of the Eight Immortals, seen clearly in the style of Chinese martial arts dedicated to his memory. The style includes moves such as delivering a kick during a backflip or bending so far back that your shoulders touch the ground. He was known to be quite entertaining, often making himself invisible, drinking water from the petals of poisonous flowers, snatching birds in flight from the sky, as well as wilting flowers simply by pointing in their direction.

Zhang appears frequently in Chinese paintings and sculpture, either with the Eight Immortals or alone, and, like the other immortals, can be seen in many different common artistic mediums and everyday objects. He may be depicted standing or seated, but is typically shown riding his white mule, usually seated facing backwards. His emblem is a fish drum, which is a tube-shaped bamboo drum with two iron rods or mallets that he carries with him, or carrying a phoenix feather or a peach, representing immortality. Since he represents old age, in the Taoist fengshui tradition, a picture or statue of him can be placed in the home or bedroom of an elderly person to help bring them a long life and a good, natural death. A picture of him on his mule offering a descendant to a newly wed couple can also be found in Taoist nuptial chapels.

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