The Nakhichevan Autonomous Soviet Socialist Republic, abbreviated as the Nakhichevan ASSR was an autonomous republic within the Azerbaijan SSR, itself a republic within the Soviet Union. It was formed on 16 March 1921 and became a part of the Azerbaijan SSR proper on 9 February 1924.

The first flag of the Nakhichevan ASSR was introduced in 1937 and contained both Azerbaijani and Armenian text. In the 1940s, when the Azerbaijani Latin alphabet was being replaced by Cyrillic, the previous flag was replaced by a Soviet flag with the Azerbaijani Cyrillic text "Нахчыван МССР" in gold and a dark blue bar along the fess.

In 1990, it became the Nakhichevan Autonomous Republic within the Republic of Azerbaijan.

In the final years of World War I, Nakhichevan was the scene of bloodshed between Armenians and Azerbaijanis, who both laid claim to the area. By 1914, the Armenian population had decreased slightly to 40% while the Azeri population increased to roughly 60%. After the February Revolution of 1917, the region was placed under the authority of the Special Transcaucasian Committee of the Russian Provisional Government and subsequently the short-lived Transcaucasian Democratic Federative Republic (TDFR) in 1918. When the TDFR was dissolved in May 1918, Nakhichevan, Nagorno-Karabakh, Zangezur (today the Armenian province of Syunik and part of the province of Vayots Dzor), and Qazakh were heavily contested between the newly formed and short-lived states of the Democratic Republic of Armenia (DRA) and the Azerbaijan Democratic Republic (ADR). In June 1918, the region came under Ottoman occupation. The Ottomans proceeded to massacre 10,000 Armenians and razed 45 of their villages to the ground. Under the terms of the Armistice of Mudros, the Ottomans agreed to pull their troops out of the Transcaucasus to make way for the forthcoming British military presence.

Under British occupation, Sir Oliver Wardrop, British Chief Commissioner in the South Caucasus, made a border proposal to solve the conflict. According to Wardrop, Armenian claims against Azerbaijan should not go beyond the administrative borders of the former Erivan Governorate (which under prior Imperial Russian rule encompassed Nakhichevan), while Azerbaijan was to be limited to the governorates of Baku and Elisabethpol. This proposal was rejected by both Armenians (who did not wish to give up their claims to Qazakh, Zangezur and Nagorno-Karabakh) and Azeris (who found it unacceptable to give up their claims to Nakhichevan). As disputes between both countries continued, it soon became apparent that the fragile peace under British occupation would not last.

In December 1918, with the support of Azerbaijan's Musavat Party, Jafargulu Khan Nakhichevanski declared the Republic of Aras in the Nakhichevan uyezd of the former Erivan Governorate assigned to Armenia by Wardrop. The Armenian government did not recognize the new state and sent its troops into the region to take control of it. The conflict soon erupted into the violent Aras War. British journalist C.E. Bechhofer described the situation in April 1920:

You cannot persuade a party of frenzied nationalists that two blacks do not make a white; consequently, no day went by without a catalogue of complaints from both sides, Armenians and Tartars [Azeris], of unprovoked attacks, murders, village burnings and the like. Specifically, the situation was a series of vicious cycles.

By mid-June 1919, Armenia had established control over Nakhichevan and the whole territory of the self-proclaimed republic. The fall of the Aras republic triggered an invasion by the regular Azerbaijani army and by the end of July, Armenian troops were forced to leave Nakhichevan City to the Azeris. Again more violence erupted, leaving some 10,000 Armenians dead and 45 Armenian villages destroyed. Meanwhile, feeling the situation to be hopeless and unable to maintain any control over the area, the British decided to withdraw from the region in mid-1919. Still, fighting between Armenians and Azeris continued and after a series of skirmishes that took place throughout the Nakhichevan district, a ceasefire was agreed. This lasted only briefly, and by early March 1920, more fighting broke out, primarily in Karabakh between Karabakh Armenians and Azerbaijan's regular army. This triggered conflicts in other areas with mixed populations, including Nakhichevan.