《成吉思汗法典》（蒙古语：ᠶᠡᠬᠡ ᠽᠠᠰᠠᠭ，鲍培转写：Yehe Zasag，西里尔字母：Их засаг），即大扎撒，是世界上第一套应用范围最广泛的成文法典，同时也是世界上最早的宪法性文档，于1206年由蒙古帝国的成吉思汗颁布实施。法典将行政权及司法权分开，创建了一套有部落民主色彩的君主政体制度。这种以两权制约的判例制度，比英国19世纪的判例制度早约600年。

不过，《成吉思汗法典》的古本于元末明初的战乱入面已经散失，结果失传了600多年。虽然法典的不少部份仍然可以零碎地在多份文献内找到，但由于这些文献数量和种类均很多，又涉及英文、古体蒙古文、现代蒙古文、汉文等达8种文本，所以要进行研究具有很高难度，结果一直未有学者或者研究机构可以完整重构这一部著作。

Yassa (alternatively: Yasa, Yasaq, Jazag, Zasag, Mongolian: Их засаг, Yehe Zasag) was a secret written code of law created by Genghis Khan. The word Yassa translates into "order" or "decree". It was the de facto law of the Mongol Empire even though the "law" was kept secret and never made public. The Yassa seems to have its origin as decrees issued in wartime. Later, these decrees were codified and expanded to include cultural and life-style conventions. By keeping the Yassa secret, the decrees could be modified and used selectively. It is believed that the Yassa was supervised by Genghis Khan himself and his stepbrother Shihihutag who was then high judge (in Mongolian: улсын их заргач) of the Mongol Empire. Genghis Khan appointed his second son Chagatai (later Chagatai Khan) to oversee the execution of the laws.