彼得·卢德维格·梅德尔·西罗(Peter Ludwig Mejdell Sylow，国际音标：/peːtær lʉdviː meːdəl syːlɯːʋ/，1832年12月12日－1918年9月7日）是挪威数学家，证明了群论中重要的基础定理，就是西罗定理。西罗出生和逝世都在克里斯蒂安尼亚（今奥斯陆）。

西罗就读于克里斯蒂安尼亚大学，在1852年取得数学学位。从1858年到1898年在腓特烈斯哈尔（今哈尔登）担任学校教师。1862年他在克里斯蒂安尼亚大学当代课讲师，教授伽罗瓦理论。当时他提出的问题，引导他发现西罗子群和其定理。1872年他发表了三条定理，现在称为西罗定理。当时没有给出证明，十年后他提供了证明。

Peter Ludwig Mejdell Sylow (IPA: [ ˈsyːlɔv]) (12 December 1832 – 7 September 1918) was a Norwegian mathematician who proved foundational results in group theory. He was born and died in Christiania (now Oslo).

Sylow was a high school teacher in Halden, Norway, from 1858 to 1898, and a substitute lecturer at Christiania University in 1862, covering Galois theory. It was then that he posed the question that led to his theorems regarding Sylow subgroups. Sylow published the Sylow theorems in 1872, and subsequently devoted eight years of his life, with Sophus Lie, to the project of editing the mathematical works of his countryman, Niels Henrik Abel.