Zhāng Guǒlǎo 張果老 (more formally Zhāng Guǒ 張果, “Old Zhāng Guǒ” being his cult-name) is a legendary Táng-dynasty Daoist immortal, traditionally counted among the Bā xiān 八仙 (“Eight Immortals”) of later popular religion. The historical figure on which the legend is based was active in the early eighth century: he is said to have refused summons from Empress Wǔ Zétiān 武則天, was persuaded by Xuánzōng 玄宗 to come to court at Lùoyáng 洛陽 in 733, was given the honorific Tōngxuán xiānshēng 通玄先生, and a temple was raised on Mount Héng 恆山 (Shānxī) in his memory. Both the Jiù Tángshū 舊唐書 and the Xīn Tángshū 新唐書 carry biographies in their Fāngjì zhuàn 方伎傳 chapters; he also figures prominently in Daoist hagiographies such as the Tàipíng guǎngjì 太平廣記.
In the alchemical literature of the Sòng he is converted into the eponymous author of various neidan texts, including [[KR5a0226|DZ 225 Tàishàng jiǔyào xīnyìn miàojīng]], whose preface attributes the work to “the Immortal Zhāng Guǒlǎo” (仙人張果老). The attribution is pseudepigraphic — the technical vocabulary and the doctrinal framework of DZ 225 (the sānhuǒ 三火, sānyáng 三陽, liànshén hédào 鍊神合道 etc.) are characteristic of the Northern-Sòng ZhōngLǚ 鍾呂 school of inner alchemy, several centuries later than the historical Zhāng Guǒ. CBDB has multiple entries indexed under “張果” but no confident lifedate match for the immortal as such.
A separate identification of “the immortal Zhāng Guǒ” with the Táng Tóngxuán xiānshēng is also implied by the title sometimes given to the neidan tradition’s author.