勾玉又称曲玉，是日本、朝鲜古代的一种首饰，呈月牙状，有首尾之分，首端宽而圆，有一钻孔，可系绳，尾端则尖而细。常见材质大多为翡翠、玛瑙、水晶、滑石等，偶见有陶土制品，但流传至今的不多，陶土材质不耐久存也是主因之一。

勾玉的形状由来主要有2说：

以目前的考古证据来看，勾玉在日本的起源最迟至少可追溯至绳文时代，而自其后的古坟时代开始已成为权势的象征之一。天皇三神器中的八阪琼曲玉即为勾玉。在朝鲜半岛，曲玉在一些新石器时代及青铜时代考古遗址皆有发现，常见于支石墓作为陪葬，曲玉在当时是权力和名望的象征。当地最著名的曲玉是三国时代新罗国王的金王冠、耳环、项链、腰带等镶着的曲玉，新罗、百济的君主都有以曲玉作为陪葬品。

Magatama(勾玉), less frequently (曲玉), are curved, comma-shaped beads that appeared in prehistoric Japan from the Final Jōmon period through the Kofun period, approximately ca. 1,000 BC to the 6th century AD. The beads, also described as jewels, were made of primitive stone and earthen materials in the early period, but by the end of the Kofun period were made almost exclusively of jade. Magatama originally served as decorative jewelry, but by the end of the Kofun period functioned as ceremonial and religious objects. Archaeological evidence suggests that magatama were produced in specific areas of Japan and were widely dispersed through the entirety of the Japanese archipelago by trade routes.