Liu Penzi 劉盆子 (10 CE-) was the first emperor that the peasant rebels of the Red Eyebrows 赤眉 set up during the end of Wang Mang's 王莽 (r. 8-23 CE) usurpation of the throne of the Han dynasty 前漢 (206 BCE-8 CE).
Liu Penzi was a descendant of Liu Zhang 劉章, Prince of Chengyang 城陽, a grandson of the dynastic founder Emperor Gaozu 漢高祖 (r. 206-195 BCE). In 25 CE, when the rebellion of the Red Eyebrows began, their leaders decided that a member of the house of Liu had to be proclaimed as emperor. Liu Penzi agreed and adopted the reign title of Jianshi 建世. His brother Liu Gong 劉恭 several times warned him and tried to pursue him to give up the title of emperor, but the main rebel leader Fan Chong 樊崇 did not allow it. When the rebels entered the capital Chang'an 長安 (modern Xi'an 西安, Shaanxi), the local magnates hid the grain reserves, so that the rebel troops were threatened by starvation.
At the end of 26 CE they therefore left the metropolitan area and withdrew to the east. Soon, they marched into a trap laid by Liu Xiu 劉秀 (the eventual Emperor Guangwu 漢光武帝, r. 25-57 CE) at Yaodi 崤底 (modern Mianchi 澠池, Henan) and were heavily defeated. Fan Chong urged Liu Penzi to surrender, and he was in fact the only one of the rebel leaders who was spared execution.
He was allowed to serve as a court gentleman (langzhong 郎中) at the court of the Prince of Zhao 趙, an uncle of Emperor Guangwu. It is unknown where and when Liu Penzi died.