King Onjo (r. 18 BC - 28 AD) is the legendary founder and first king of the ancient Korean kingdom of Baekje. He is known as the third son of King Dongmyeong, the founder of Goguryeo. King Dongmyeong had three sons; Yuri, Biryu, and Onjo. Afer Yuri, born from King Dongmyeong's first wife in Fuyu, came to Goguryeo and became the heir of throne, Biryu and Onjo left Goguryeo and moved south to found their own kingdoms.

According to the Samguk Yusa, Biryu (비류/沸流) founded his kingdom in Michuhol (미추홀/彌鄒忽), but his didn't last long. The modern-day location of Michuhol is in debate—it was identified as Incheon (a city near Seoul) for a long time, but recently it was suggested as Incheon County, City of Asan, South Chungcheong. Onjo found his kingdom in Wirye, in or near modern-day Seoul, and named it Sipje (십제/十濟). Biryu's people joined Sipje after Biryu's death and King Onjo renamed it to Baekje.

Whether the legend tells the truth or not, it indicates that Buyeo, Jolbon Buyeo (or Goguryeo), and South Buyeo (or Baekje) shared a common origin.