Guo Si 郭汜 (d. 197), also called Guo Duo 郭多, was a military leader in the late Eastern Han period 東漢 (25-220 CE).
He hailed from Zhangyi 張掖 (modern Zhangyi, Gansu). After the death of the usurper Dong Zhuo 董卓, Guo Si and Li Jue 李傕 entered the capital Chang'an 長安 (modern Xi'an 西安, Shaanxi), where they killed a lot of Dong's adherents, among others the Minister of Education (situ 司徒, see Three Dukes 三公) Wang Yun 王允. Guo was made General of the rear (hou jiangjun 後將軍) and was given the title of Marquis of Meiyang 美陽侯.
He usurped the power of the central government, together with Li Jue and Fan Chou 樊稠, but the triumvirate soon fell apart, and Fan Chou was killed by the two others. Guo Si and Li Jue fought against each other in the streets of Chang'an.
Li Jue took Emperor Xian 漢獻帝 (r. 189-220) as a hostage, while Guo Si was able to seize the highest ministers. Zhang Ji 張濟 finally managed to conclude an armistice, so that Li Jue released the emperor. Guo Si suggestd that Emperor Xian might take his seat in Mei 郿 because both capitals, Luoyang 洛陽 (modern Luoyang, Henan) and Chang'an, were thoroughly devastated, but the Emperor wished to return to Luoyang and left for the military camp of Yang Feng 楊奉.
The two regents pursued the emperor until they arrived in Caoyang 曹陽. They defeated the troops of Yang Feng, but with a group of a few dozen of retainers Emperor Xian was able to escape. Guo Si ws finally killed by Wu Xi 五習, one of his generals.