祖己（？ - ？），又作孝己，是商朝武丁的嫡長子，以其苦孝有名。

殷墟甲骨卜辭作且己，又作「小王」、一期卜辭作「兄己」，康丁卜辭作「小王父己」，帝乙、帝辛卜辭作「且己」（祖己），《史記·殷本紀》稱為祖己，《帝王世紀》作孝己，《太平御覽》、《帝王世紀》作為武丁嫡長子。

《史記·殷本紀》、《尚書·高宗肜日》記載「帝武丁祭成湯，明日，有飛雉登鼎耳而呴，武丁懼。祖己曰：『王勿憂，先修政事。』」武丁遂「修政行德」，商朝再度復興。

《太平御覽》卷八十三載孝己母親（《尚書·毋逸》《竹書紀年》作妣戊）早死，繼母妣辛虐待孝己。並捏造事實、誹謗孝己。父親武丁聽信，流放孝己，餓死于野外，妣辛生祖庚（且庚）即武丁位為商王。

殷人周祭祀譜中，祖己位居武丁之後，祖庚之前。郭沫若、韓江蘇等認為祖己被立為太子，但先武丁而死，所以受到殷人的周祭。江林昌認為祖己曾經稱王。

Zu Ji was the eldest son of King Wu Ding but despite his kingly title he never succeeded his father as a Shang dynasty King of China. In later texts he is known as Xiao Ji, while contemporary inscriptions record his name as Jie.

In the Records of the Grand Historian he was said by Sima Qian to have died in distant exile during the twenty-fifth year of his father's reign.

Oracle script inscriptions on bones unearthed at Yinxu confirm his lineage and his failure to succeed to the throne.