The Sisak People's Liberation Partisan Detachment, also known as the 1st Sisak Partisan Detachment (Serbo-Croatian: Sisački narodnooslobodilački partizanski odred, 1. Sisački partizanski odred) was the first armed anti-fascist resistance unit formed by a resistance movement in occupied Yugoslavia and Europe during World War II. This first detachment of the Yugoslav Partisans was established in occupied Yugoslavia, in the Brezovica forest near Sisak (in today's Croatia) on June 22, 1941, the day Germany invaded the Soviet Union. It had 79 members, mainly Croats with the exception of one notable Serb woman, Nada Dimić, and was commanded by Vladimir Janjić-Capo.
This event marked the start of armed anti-fascist resistance in occupied Yugoslavia. Today, June 22 is commemorated in Croatia every year as a public holiday called the Anti-Fascist Struggle Day.
Janko Bobetko, who 50 years later became one of the most prominent Croatian generals in Croatian War of Independence, was one of the founding members of this unit.