Yi Saek, also known by his art name Mogŭn (목은), was a Korean writer and poet. His family belonged to the Hansan Yi clan. Yi Saek played a crucial role in the introduction and localisation of philosophy of Zhu Xi. He studied Neo-Confucianism in Yuan Dynasty China and opened an academy after his return to Goryeo, and from his academy the founders of Joseon Dynasty were educated.