King Onjo (Hangul/Hanja: 온조왕/溫祚王; ?-28 CE; Reigned: 18 BCE–28 CE) was the founding monarch of Baekje (백제/百濟), one of the Three Kingdoms of Korea. According to the Samguk Sagi (삼국사기/三國史記), he was the ancestor of all Baekje kings and progenitor of the Royal House of Buyeo (부여/扶餘) which is later changed its surname and designated ancestral seat: Buyeo Seo Clan (부여 서씨/扶餘徐氏) - originated from Baekje's Last Stand; Buyeo County a.k.a Sabi, Southern Chungcheong Province.
He was the third son of King Chumo-Dongmyeong (Jumong), the founder of the northern Korean kingdom Goguryeo. He was the younger half-brother of Yuri-myeong, who became Goguryeo's second king, and younger brother of Biryu, described in some records as the founder of Baekje.
Dongmyeong had three sons: Yuri-myeong, Biryu, and Onjo. When Yuri-myeong, born from Dongmyeong's previous wife in Dongbuyeo which was Lady Ye, 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 at Michuhol (미추홀/彌鄒忽) but his didn't last long. The modern-day location of Michuhol is in debate—it has been identified as Incheon Metropole for a long time, but recently it is suggested as Asan, Southern Chungcheong Province. Onjo found his kingdom in Wiryeseong (present-day Seoul Special City), and named it Sipje (십제/十濟).
Biryu's people joined Sipje after Biryu's death and Onjo renamed it to Baekje. After that, the capital city of Baekje was moved southward from Habuk Wiryeseong because the Malgal were located at the North and Nangnang was located to the East. Both capital cities correspond to land within current Seoul. In 3 BCE and 8 BCE, the Malgal tribes attacked from the north, and both times, Onjo directly led his armies and won victories over the invaders. In 5 BCE, Onjo moved the capital city to a more defensible location south of the Han River, renaming it Hanam Wiryeseong, and sent a messenger to the king of the Mahan confederacy telling him of the recent action.
At this time, Onjo already had plans of conquering Mahan and Jinhan. By 7 CE, he was already preparing his armies for war and finally attacked during 8 CE. In 8 CE, he secretly took his armies across the border, laying the deceit that he was going to hunt in the forests. Soon, all of Mahan except for two fortresses were conquered. 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.
Baekje had expanded greatly, and several fortresses were being built every year. Peace lasted for 8 years until 16 CE, 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 CE, but were once again defeated by Onjo and his army.
King Onjo died of natural causes in 28 CE, during the 46th year of his reign. He was succeeded by his eldest son, Daru. King Onjo laid the foundations for a powerful dynasty that would last for 678 years and 31 rulers (Downfall: 660 CE - during the reign of King Uija).