Sima Zhen (司馬貞; 679–732), courtesy name Zizheng (Tzu-cheng; 子正), was a Tang dynasty Chinese historian born in what is now Jiaozuo, Henan.

Sima Zhen was one of the most important commentators on the Shiji. His commentary is known as the Shiji Suoyin, which means "Seeking the Obscure in the Records of the Grand Historian".

司馬貞（679年 - 732年），字子正，河內郡（今河南省沁陽）人，唐朝官員、歷史學家。

生平沒有詳細記載。僅知司馬貞官至朝散大夫、國子博士及弘文館學士。開元時，任潤州別駕。編纂有《史記索隱》三十卷。重要的史記集結者。《史記索隱》在開元二十年（732年）左右成書，成書後不久司馬貞逝世，年約五十三歲。