Onjo (; ?–28, reigned c. 18 BC – AD 28) was the founding monarch of Paekche, one of the Three Kingdoms of Korea. According to the Samguk sagi, he founded the royal family of Paekche.
Background
There are a few theories and legends of Onjo's parentage. One is that he was the third son of King Dongmyeong (Jumong), the founder of the northern Korean kingdom Goguryeo. He was the younger brother of Yuri, who became Goguryeo's second king, and younger brother of Biryu who built small state in Michuhol.
The second theory is that he is the son of Wutae, his mother's first husband. A third legend says that his older brother Biryu was his mother's son with Wutae but Onjo was born after the second marriage with King Dongmyeong.
Founding and expansion of Paekche
Dongmyeong had three sons: Yuri, Biryu, and Onjo. When Yuri, born from Dongmyeong's previous wife in Dongbuyeo, came to Goguryeo and became the heir to the throne, Biryu and Onjo moved south to found their own kingdoms. According to the Samguk yusa, Biryu founded his kingdom in Michuhol (), but his didn't last long. The citizens of the last two Mahan fortresses surrendered to Onjo and were given mercy. The Mahan King, however, committed suicide and left a letter for Onjo, asking him to take in Mahan's people with kindness and mercy. Onjo respected the Mahan King's last request and took his people in.
However, according to a recent study, Paekche conquered Mahan's Mokji after the mid-3rd century. In fact, it is presumed to be an article written for the founder about the events of later generations.
Paekche had expanded greatly, and several fortresses were being built every year. Peace lasted for 8 years until 16 AD, when a former Mahan general caused a rebellion. Onjo directly led an army of 5,000 and successfully destroyed the rebellion. Soon after this, the Malgal tribes invaded again during 22 AD, but were once again defeated by Onjo and his army.