Jeong Mong-ju (1337 – 1392), also known by his pen name Po-Eun (Korean: 포은), was a Korean civil minister and scholar from the end period of the Goryeo Dynasty. Jeong Mong-ju was born in Yeongcheon, Gyeongsang province to a family from the Yeongil Jeong clan. At the age of 23, he took three different civil service literary examinations (Gwageo) and received the highest marks possible on each of them. In 1367, he became an instructor in Neo-Confucianism at the Gukjagam, then called "Seonggyungwan", whilst simultaneously holding a government position, and was a faithful public servant to King U.

The king had great confidence in his wide knowledge and good judgement, and so he participated in various national projects and his scholarly works earned him great respect in the Goryeo court. In 1372, Jeong Mong-ju visited China, as a diplomatic envoy. Around the time, as waegu (Japanese pirate)'s invasions to the Korean Peninsula were extreme, Jeong Mong-ju was dispatched as a delegate to Kyūshū in 1377. His negotiations led to promises of Japanese aid in defeating the pirates. He traveled to the Chinese capital city in 1384 and negotiations with the Ming Dynasty led to peace with China in 1385. He also founded an institute devoted to the theories of Confucianism.

Jeong Mong-ju was murdered in 1392 by five men on the Sonjukkyo Bridge in Gaeseong following a banquet held for him by Yi Bang-won (later Taejong of Joseon), the fifth son of Yi Seong-gye, who overthrew the Goryeo Dynasty, in order to found the Joseon Dynasty. Jeong Mong-ju was murdered because he refused to betray his loyalty to the Goryeo Dynasty. Yi Bang-won recited a poem to dissuade Jeong Mong-ju from remaining loyal to the Goryeo court, but Jeong Mong-ju answered with another poem (Dansimga, 단심가/ 丹心歌) that affirmed his loyalty.

Yi Bang-won's poem

하여가 (何如歌)
이런들 어떠하리 저런들 어떠하리 此亦何如彼亦何如 (차역하여피역하여)
만수산 드렁칡이 얽어진들 어떠하리 城隍堂後垣頹落亦何如 (성황당후원퇴락역하여)
우리도 이같이 얽어져 백년까지 누리리라 我輩若此爲不死亦何如 (아배약차위불사역하여)
Does it matter, whether you go this way or that?
The Mansu arrowroots are tangled together, this way:
Tangled likewise, let us prosper a hundred years.

Jeong Mong-ju's sijo (Dansimga)

단심가 (丹心歌)
이몸이 죽고 죽어 일백 번 고쳐 죽어 此身死了死了一百番更死了 (차신사료사료일백번갱사료)
백골이 진토되어 넋이라도 있고 없고 白骨爲塵土魂魄有無也 (백골위진토혼백유무야)
임 향한 일편 단심이야 가실 줄이 있으랴 鄕主一片丹心寧有改理歟 (향주일편단심유개리여)
Though I die and die again a hundred times,
That my bones turn to dust, whether my soul remains or not,
Ever loyal to my Lord, how can this red heart ever fade away?

Yi Seong-gye is said to have lamented Jeong Mong-ju's death and rebuked his son because Jeong Mong-ju was a highly regarded politician by the courts of China and Japan. The bridge where Jeong Mong-ju was murdered, now in North Korea, has now become a national monument of that country. A brown spot on one of the stones is said to be Jeong Mong-ju's bloodstain, and is said to become red whenever it rains. Currently, his direct surviving descendants are his 21st and 22nd generation, all of whom reside in South Korea and the United States.

The 474-year-old Goryeo Dynasty symbolically ended with Jeong Mong-ju's death, and was followed by the Joseon Dynasty for 505 years (1392 – 1897). Jeong Mong-ju's noble death symbolises his faithful allegiance to the king, and he was later venerated even by Joseon monarchs. In 1517, 125 years after his death, he was canonised into the National Academy alongside other Korean sages such as Yi I (Yulgok) and Yi Hwang (Toegye).

The 11th pattern of ITF Taekwon-Do is named after Poeun. The pattern is performed as part of the testing syllabus for the level of 2nd degree black belt. The diagram ( - ) represents his unerring loyalty to the king and country towards the end of the Goryeo Dynasty.