Leon Petrazycki (Polish: Leon Petrażycki; Russian: Лев Иосифович Петражицкий Lev Iosifovich Petrazhitsky; 13 April 1867, Vitebsk Governorate – 15 May 1931, Warsaw) was a Polish philosopher, legal scholar and sociologist. He is considered one of the important forerunners of the sociology of law.

Leon Petrażycki was born into the Polish gentry of the Vitebsk Governorate in the Russian Empire. He graduated from Kiev University in 1890, spent two years on a scholarship in Berlin and got his doctorate in 1896 from the University of St. Petersburg. At the latter university he served from 1897 to 1917 as a professor of the Philosophy of Law.

In 1906 Petrażycki was elected to the ill-fated First Duma as a member of the Constitutional Democratic Party. When the legislature was dissolved after a few months, he was convicted and incarcerated for his protests. He was appointed to the Supreme Court of Russia in 1917, but had to flee the country when the Bolshevik revolution succeeded. He found a new home in Poland and became the first professor of Sociology at Warsaw University in 1919.