The National Defense Organization "Carpathian Sich" (Ukrainian: Організація народної оборони «Карпатська Січ», romanized: Orhanizacija narodnoï oborony «Karpats'ka Sič», also known as the Carpathian Sich) were Ukrainian nationalist militia soldiers of the short-lived state of Carpatho-Ukraine. They resisted the Hungarian invasion of Carpatho-Ukraine.

The Carpathian Sich was formed on 9 November 1938. It was formed in opposition to the newly elected moderate Ukrainian nationalist prime minister of the Subcarpathian Autonomous Region within Czechoslovakia, Avgustyn Voloshyn. The Carpathian Sich was based in Khust which became a temporary capital since Uzhhorod, Mukacheve, and Berehove became part of Hungary according to the First Vienna Award. The organization was led by Dmytro Klympush and Ivan Rohach as a deputy of the first and performed paramilitary and police duties in adjustment with Czechoslovak armed forces. Although the Carpathian Sich's leaders were local Transcarpathian Ukrainian nationalists, most of its forces consisted of Ukrainian activists who crossed over the mountains from Galicia.

Inspired by Nazi German stormtroopers, the Carpathian Sich terrorized Jews and segments of the population that were pro-Russian: elements that the organization considered politically suspect.

On 22 January 1939, in Khust, the 20th anniversary festivities commemorating the 1919 Act Zluky (Act of Union) were held. On 13 March 1939, as Czechoslovakia fell apart, according to historians Paul R. Magocsi and Ivan Pop, the Sich prepared a coup against the Carpatho-Ukrainian government of Avgustyn Voloshyn with the encouragement of Nazi Germany's Schutzstaffel (SS) and attacked Czechoslovak troops. This resulted in the deaths of several Czechoslovak soldiers and many Sich members.

On 15 March 1939, Hungary invaded Carpatho-Ukraine. Avgustyn Voloshyn fled to Romania along with the Czech military rather than fight the invading Hungarian forces. The poorly armed forces of Carpatho-Ukraine, along with unarmed high school students and religious seminary students, were sent out by the Carpathian Sich; they were no match for the Hungarian military, which by March 18, 1939, had captured all of Carpatho-Ukraine. During these three days of fighting, hundreds of Carpathian Sich members died in battle while they slowly retreated until they came into conflict with Polish border guards; most died on the other side of the Carpathians.

In September 1939, 600 veterans of the Sich were authorized by Nazi German intelligence (the Abwehr) to form a battalion-sized combat unit that participated in the German invasion of Poland. It was disbanded soon afterwards.

During the 20th and 21st centuries, post-Communist Ukraine, Ukrainian nationalists, and pro-Ukrainian émigré authors in the West have organized events celebrating the Carpathian Sich as a symbol of bravery and national pride against foreign occupation.

Carpathian Sich (Karpatska Sich) (also the Carpathian Sich National Defense Organization) was a paramilitary organization in Carpatho-Ukraine formed from units of the Ukrainian National Defense (organized in Uzhhorod by Ukrainian nationalists and headed by Stepan Rosokha). The leadership of the Carpathian Sich consisted of the command (commander, D. Klempush; deputy-commander, I. Roman) and the staff of officers. The organization's headquarters were in Khust, and there were 10 individual district commands with subordinate local sections, each of which conducted military and political training of several thousand men.