世界之树（古诺尔斯语：Yggdrasill），又称为「宇宙树」或「乾坤树」，在北欧神话中，这个巨木的枝干构成了整个世界。

其音译名为「尤克特拉希尔」。所谓「尤克」表示令人害怕之意，也是主神奥丁的另一个名字。而「特拉希尔」是指马，因此组合起来就是「奥丁的马」。其由来是因为奥丁曾经把自己吊在树上，并用长枪—昆古尼尔刺伤自己，因而发现了卢恩字母。奥丁将此举说是「骑马」，世界之树因此有了尤克特拉希尔之名。

此树的树种是白蜡树（Fraxinus），高达天际。于此树上衍生有九个王国：

其下有三根粗大的根：

Yggdrasil (/ˈɪɡdrəsɪl/ or /ˈɪɡdrəzɪl/; from Old Norse Yggdrasill, pronounced [ˈyɡːˌdrasilː]) is an immense mythical tree that connects the nine worlds in Norse cosmology.

Yggdrasil is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and the Prose Edda, written in the 13th century by Snorri Sturluson. In both sources, Yggdrasil is an immense ash tree that is central and considered very holy. The gods go to Yggdrasil daily to assemble at their things. The branches of Yggdrasil extend far into the heavens, and the tree is supported by three roots that extend far away into other locations; one to the well Urðarbrunnr in the heavens, one to the spring Hvergelmir, and another to the well Mímisbrunnr. Creatures live within Yggdrasil, including the wyrm (dragon) Níðhöggr, an unnamed eagle, and the stags Dáinn, Dvalinn, Duneyrr and Duraþrór.