During the Joseon Dynasty, medicinal foods derived from herbs were often more effective than traditional medicines. In addition, the royal family of the Joseon Dynasty believed that foods could be used as various disease treatments. Grain-based foods, especially medicinal porridges (藥粥), were most frequently used for diet therapy. We investigated various types of diet-related diseases suffered by King Injo (仁祖) as well as how the diseases were treated using medicinal porridges based on information in the SeungjeongwonIlgi (承政院日記), which is the daily record of the Royal Secretariat of the Joseon Dynasty. This study examined the SeungjeongwonIlgi of King Injo from his1st year (1623) to 27th year (1649) on a website database maintained by the National Institute of Korean History. According to the records, King Injo suffered from severe diarrhea several times due mainly to febrile disease (煩熱症) as well as abdominal dropsy (脹滿) throughout his entire life. Major diseases affecting King Injo were due to his unhealthy eating habits and psychological factors. For treatment, royal doctors prescribed around 15 medicinal porridges, including nelumbo (seed) porridge (Yeonja-juk), milk porridge (Tarak-juk), Chinese dioscorea porridge (Sanyak-juk), mungbean porridge (Nokdu-juk), perilla seed porridge (Imja-juk), adzuki-bean porridge (Pat-juk), soybean porridge (Kong-juk), Korean-leek porridge (Buchu-juk), and so on, in addition to other medical treatments. Diet therapy using medicinal porridges has been used throughout history since the Joseon Dynasty period. However, knowledge of traditional diet therapy and medicinal porridges used by monarchs in the Joseon Dynasty is insufficient. Therefore, in-depth study is needed to understand the theory of traditional medicinal foods as well as explore their application to patients in the context of modern medicine.