The Republic of Mountainous Armenia (Armenian: Լեռնահայաստանի Հանրապետութիւն, romanized: Leṙnahayastani Hanrapetut’yun), also known as simply Mountainous Armenia (Լեռնահայաստան, Leṙnahayastan), was an anti-Bolshevik Armenian state roughly corresponding with the territory that is now the present-day Armenian provinces of Vayots Dzor and Syunik, and some parts of the present-day Republic of Azerbaijan (in particular, Nakhchivan Autonomous Republic) in the west. It was established by military commander and Armenian political thinker Garegin Nzhdeh and his allies with the support of local guerrilla forces, following the suppression of the February Uprising in April 1921. It was not recognized by any country but existed until mid-July of the same year.

In 1920–1921 with an Armenian Citizen Army of around 15,000 General Nzhdeh and his highly motivated soldiers inflicted heavy casualties upon the Kemalist Turkish army of over 100,000 coming from the West (Ankara) and the Soviet Red Army with over 150,000 forces coming from the East (Baku).

Following World War I, the signing of the Treaty of Sèvres, and in the ensuing peace negotiations in Paris, the Allies had vowed to punish the Young Turks and reward some, if not all, of the eastern provinces of the empire to the nascent Armenian Republic. However, the Allies were more concerned with concluding the peace treaties with Germany and the other European members of the Central Powers. In matters related to the Near East, the principal western powers, Great Britain, France, Italy and the United States, had conflicting interests over the spheres of influence they were to assume. While there were crippling internal disputes between the Allies, the United States was reluctant to accept a mandate over Armenia. Meanwhile, the recently formed Russian Soviet Federative Socialist Republic and Turkish National Movement had both set their sights on taking over the Caucasus, including Armenia. The Bolsheviks sympathized with the Turkish Movement due to their mutual opposition to the western powers, or "Western Imperialism," as the Bolsheviks referred to it. The Soviet government allied with the Turkish nationalists and sent them gold and weapons. This proved disastrous for the Armenians, and eventually Western Armenia fell to the invading forces.

Armenia gave way to communist power in late 1920. The Soviet 11th Red Army's invasion of the First Republic of Armenia started on the 29 November 1920. The actual transfer of power took place on December 2 in Yerevan, when the Armenian leadership approved an ultimatum presented to it by the Soviet plenipotentiary Boris Legran. Armenia agreed to join the Soviet sphere, while Soviet Russia agreed to protect its remaining territory from the advancing Turkish army. The Soviets also pledged to take steps to rebuild the army, to protect the Armenians and not to persecute non-communists, although the final condition of this pledge was reneged when the Dashnaks were forced out of the country. The Soviet Government proposed the annexation of the Nagorno-Karabagh and Syunik regions of Armenia to Soviet Azerbaijan. This step was strongly rejected by Garegin Nzhdeh, who declared Syunik as a self-governing region on December 25, 1920. In January 1921 Drastamat Kanayan sent a telegram to Nzhdeh, suggesting allowing the sovietisation of Syunik, through which they could gain the support of the Bolshevik government in solving the problems of the Armenian lands. Nzhdeh did not depart from Syunik and continued his struggle against the Red Army and Soviet Azerbaijan.

On 18 February 1921, the ARF led an anti-Soviet rebellion in Yerevan and seized power. The ARF controlled Yerevan and the surrounding regions for almost 42 days before being defeated by the numerically superior Red Army troops later in April 1921. The leaders of the rebellion then retreated into the Syunik region.

On 26 April 1921, the 2nd Pan-Zangezurian congress, held in Tatev monastery, announced the independence of the self-governing regions of Daralakyaz (Vayots Dzor), Zangezur, and Mountainous Artsakh, under the name of the Republic of Mountainous Armenia and later on 1 June 1921, it was renamed the Republic of Armenia.

• History of Nagorno-Karabakh
• List of historical unrecognized states and dependencies

• Հայոց Գաղափարաբանության Հենասյուները from noravank.am
• Other Armenian States from freenet.am