也先太师（蒙古语：ᠡᠰᠡᠨ
ᠲᠠᠢᠱᠢ，鲍培转写：Esen taiš，西里尔字母：Эсэн тайш；？－1454年），简称也先，清朝译作额森，第28代蒙古大汗，亦是相当于中国明朝中叶时蒙古瓦剌部领袖，向明朝朝贡被封为敬顺王，同时被脱脱不花封为太师，在他统治期间，瓦剌达到极盛。

也先的父亲脱欢统一了蒙古东部地区，并由北元的后代脱脱不花取得了可汗之位，也先在1439年继承成为首领，向东发展，征服了女真，势力到达朝鲜北境，并以明朝拒绝贸易之名进攻明朝，1449年在土木堡之变一战中，打败明军，俘虏了明英宗并包围燕京，明朝形势危急，后围攻燕京不成，退回蒙古，并释回明英宗。

Esen Taishi (Mongolian:Эсэн тайш; died 1455) was a powerful Oirat Khagan of Northern Yuan dynasty based in Mongolia in the 15th century. He is best known for capturing the Zhengtong Emperor of the Ming dynasty in 1450 after the Battle of Tumu Fortress and briefly reuniting the Mongols. The Oirats reached their peak under his rule.