Lavo Čermelj, Italianized in Lavo Cermeli (10 October 1889 – 26 January 1980) was a Slovene physicist, political activist, publicist and author. In the 1930s, he was among one of the foremost representatives of Slovene anti-Fascist émigrés from the Italian-administered Julian March, together with Josip Vilfan, Ivan Marija Čok, and Engelbert Besednjak.
Lavo Čermelj was born in Trieste, then part of the Austro-Hungarian Empire. After finishing the German language lyceum in his native town, he enrolled in the Charles University in Prague, where he studied law for one year. He then switched to the University of Vienna, where he studied mathematics and physics, graduating from physics in 1914. During World War I he was drafted in the Austro-Hungarian Army. After the war he returned to Trieste, then already part of the Kingdom of Italy, where he worked as a professor at a private Slovene language high school. In the late 1920s he collaborated with several underground organizations that were resisting the policies of Fascist Italianization in the Julian March. When his activities were traced by the Italian Fascist secret police, he illegally emigrated to the Kingdom of Yugoslavia. He settled in Ljubljana, and got employed at the Bežigrad Grammar School.