张元素1151年 - 1234年，字洁古，晚号洁古老人，金代易州（即今河北省易县）人，著名中医师

，深通药理学，善医伤寒，善于化裁古方，以及善于创新。八岁试童子举，二十七岁试经义进士。因为犯了庙讳而落第，弃仕学医。

张元素约与金代另一名医刘完素同时代。刘完素病伤寒，八日不愈，头痛脉紧，令他不知所措。张元素前去探望，刘完素看不起他，面壁不顾。张元素不但断准其病，且指出用药不当，致使刘完素信服。后按张元素意见用药遂愈，自此张氏名声大振。

张氏深入研究《内经》等医学经典，并且学习了张仲景、王叔和、孙思邈、钱乙等人的医学，完善了中药升降浮沉理论，开创了金元时期的「易水学派」。他对药物气味的升降作用，和药物的归经，有独特见解。

他的代表著作有《医学启源》三卷、《珍珠囊》一卷、《脏腑标本寒热虚实用药式》、《洁古刺诸痛法》、《医方》、《药注难经》、《洁古本草》等等。

其子张璧也是医学家。

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Zhang Yuansu 张亓素 (a.k.a. Zhang Jiegu; ca. 1151–1234) was one of the most historically influential Traditional Chinese medicine physicians in the period of transition from China's northern Jin dynasty to the Mongolian Yuan dynasty.

Zhang Jiegu integrated medicinal materials into the five element framework (Wuxing) with both the five shen herbs (spirit herbs) framework and qi meridians. He helped to more clearly define the association of the "tastes" of medicinals and their believed effect on the different organ systems. Zhang asserted that herbs entered into and influenced the meridians. The culmination of Zhang's work was a book called Bag of Pearls (Zhenzhu Nang 珍珠囊).

According to Zhang Jiegu: