|Inspector of Jiao Province (交州刺史)|
c. 271 –?
|Succeeded by||Tao Huang|
|Champion General (冠軍將軍)|
|Occupation||Military general, politician|
|Courtesy name||Shihong (世洪)|
|Peerage||Marquis of Yuyao (餘姚侯)|
Yu Si was the fourth son of Yu Fan, an official who served under Sun Quan, the founding emperor of Eastern Wu, and under Sun Quan's predecessor, Sun Ce. His ancestral home was in Yuyao County (餘姚縣), Kuaiji Commandery, which is in present-day Yuyao, Zhejiang. However, he was born in Nanhai Commandery (南海郡; around present-day Guangzhou, Guangdong) in Jiao Province because his father was exiled there by Sun Quan for showing disrespect on several occasions. Yu Si was 15 years old when his father died. After Yu Fan's death, Sun Quan freed his family from exile and allowed them to return to Yuyao County and bring Yu's remains back there for burial.
In 258, the Wu regent Sun Chen deposed the emperor Sun Liang and replaced him with Sun Xiu, the Prince of Langya (琅邪王). Before Sun Xiu arrived at the palace for the coronation, Sun Chen wanted enter the palace first and behaved in a manner as if he was going to usurp the throne. He then summoned the court officials for a meeting. All of them looked fearful except for Yu Si, who appeared calm and composed. Yu Si told Sun Chen: "My lord, you hold an important position in the state just like Yi Yin and the Duke of Zhou in the past. You also wield the authority to depose and enthrone emperors. You will be bringing peace to the spirits of the past rulers and bringing benefits to the people. Everyone is very excited about this. It is as if Yi Yin and Huo Guang have returned from the dead. However, now, you wish to enter the palace before the Prince's arrival. This will cause instability and make everyone feel suspicious of you. This isn't the way for you to establish a good reputation as a loyal and filial subject." Sun Chen felt unhappy after hearing Yu Si's words, but he nonetheless honoured Sun Xiu as the new emperor.
After Sun Xiu ascended the throne, he appointed Yu Si, He Shao, Wang Fan and Xue Ying as Central Regular Mounted Attendants (散騎中常侍). Later, during the reign of Sun Hao, Yu Si was appointed as an Ambassador-Inspector of the Army (監軍使者) and was ordered to lead Wu forces to attack Fuyan (扶嚴). For his success in the campaign, he was promoted to Champion General (冠軍將軍), appointed as the Inspector (刺史) of Jiao Province, and enfeoffed as the Marquis of Yuyao (餘姚侯). He died of illness in an unknown year.
- de Crespigny (2007), p. 994.
- (翻有十一子，第四子汜最知名， ...) Sanguozhi vol. 57.
- (虞翻字仲翔，會稽餘姚人也， ...) Sanguozhi vol. 57.
- (權積怒非一，遂徙翻交州。) Sanguozhi vol. 57.
- (會稽典錄曰：汜字世洪，生南海，年十六，父卒，還鄉里。) Kuaiji Dianlu annotation in Sanguozhi vol. 57.
- Sima (1084), vol. 77.
- (孫綝廢幼主，迎立琅邪王休。休未至，綝欲入宮，圖為不軌，召百官會議，皆惶怖失色，徒唯唯而已。汜對曰：「明公為國伊周，處將相之位，擅廢立之威，將上安宗廟，下惠百姓，大小踴躍，自以伊霍復見。今迎王未至，而欲入宮，如是，羣下搖蕩，衆聽疑惑，非所以永終忠孝，揚名後世也。」綝不懌，竟立休。) Kuaiji Dianlu annotation in Sanguozhi vol. 57.
- (... 永安初，從選曹郎為散騎中常侍，後為監軍使者，討扶嚴，病卒。) Sanguozhi vol. 57.
- (休初即位，汜與賀邵、王蕃、薛瑩俱為散騎中常侍。以討扶嚴功拜交州刺史、冠軍將軍、餘姚侯，尋卒。) Kuaiji Dianlu annotation in Sanguozhi vol. 57.
- (翻有十一子， ...) Sanguozhi vol. 57.
- Chen, Shou (3rd century). Records of the Three Kingdoms (Sanguozhi).
- de Crespigny, Rafe (2007). A biographical dictionary of Later Han to the Three Kingdoms (23–220 AD). Brill. ISBN 978-90-04-15605-0.
- Pei, Songzhi (5th century). Annotations to Records of the Three Kingdoms (Sanguozhi zhu).
- Sima, Guang (1084). Zizhi Tongjian.