司马贞（679年 - 732年），字子正，河内郡（今河南省沁阳）人，唐朝官员、历史学家。

生平没有详细记载。仅知司马贞官至朝散大夫、国子博士及弘文馆学士。开元时，任润州别驾。编纂有《史记索隐》三十卷。重要的史记集结者。《史记索隐》在开元二十年（732年）左右成书，成书后不久司马贞逝世，年约五十三岁。

Sima Zhen (司马贞; 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".