Hovhannes Kajaznuni, or Hovhannes Katchaznouni (14 February 1868 – 15 January 1938), was the first Prime Minister of the First Republic of Armenia from June 6, 1918 to August 7, 1919.

Kajaznuni was born as Hovhannes Ter-Hovhannisian in 1868 in the town of Akhaltsikhe, then part of the Russian Empire, now part of Georgia. He attended secondary school in Tiflis from 1877 to 1886. In 1887, he moved to Saint Petersburg and entered the Citizens' Architectural Institute, graduating with honors in 1893. While in Saint Petersburg, Kajaznuni joined the Armenian Revolutionary Federation, eventually becoming one of its most important figures. After graduation, he worked at the construction department of the Baku provincial administration (1893–95), as an architect in Batumi (1895–97), and as regional architect at the Tiflis provincial administration (1897–99). Between 1899 and 1906, he worked as a senior architect in Baku, designing hospitals and apartment buildings, his most notable work being the Saint Thaddeus and Bartholomew Cathedral completed in 1911. After 1906 he devoted himself to political and social activities.

Kajaznuni was forced to leave the Caucasus in 1911 to avoid being called to testify at the trial of Armenian Revolutionary Federation members mounted by the Russian government in Saint Petersburg in January 1912. He lived in Istanbul and then in Van until 1914, when he returned to the Caucasus. He became a member of the Armenian National Council in 1917 and was an ARF representative in the Seym (the Transcaucasian Parliament) until 1918.

He was part of the Armenian delegation that conducted peace talks with the Ottoman Empire at the Trebizond Peace Conference, beginning on March 14, 1918. The three groups of Transcaucasus delegates—Muslim, Georgian and Armenian—had divergent aims, and were in a weak position to negotiate with the Ottomans. While the talks progressed, the Ottoman Third Army retook Erzurum after the Imperial Russian army abandoned it and advanced to the previous frontier with Russia. These setbacks spurred Akaki Chkhenkeli, the Georgian Menshevik leader of the Transcaucasus delegation, to unilaterally inform the Ottomans that he would accept the Treaty of Brest-Litovsk as the basis for negotiation, and thereby abandon Armenian claims to portions of Ottoman territory. This concession was repudiated by the Seym, which ordered Chhenkeli and the delegation to return to Tbilisi.

The capture of Batumi by Ottoman troops on April 14, 1918 sapped the will of the Georgian Mensheviks to continue fighting the Ottomans, and they pushed their Transcaucasus allies to accept the two Ottoman prerequisites for resuming negotiations: a recognition of Turkey's territorial rights and a full break with Russia. This resulted in the Mensheviks and Muslims in the Seym proposing on April 22, 1918 to establish a Transcaucasian Democratic Federative Republic with reluctant endorsement from the increasingly isolated Armenian representatives. The new republic's cabinet was selected by Chhenkeli as premier-designate, and included Kajaznuni as one of four Armenians. One of Chhenkeli's first acts, without consulting the Seym or the Armenian cabinet members, was to order the Armenian army to surrender Kars to the Ottomans. The furious Armenian leaders tendered their resignations from the cabinet and demanded Chhenkeli be replaced. The Mensheviks would only agree to replace him with Kajaznuni or another Armenian. The Armenians realized that nominating an Armenian premier would cause the Ottomans to attack Russian Armenia, which was on the front-line since the loss of Kars. Accordingly, Kajaznuni and his fellow Dashnaks allowed the Seym to confirm their cabinet positions on April 26, 1918.

Kajaznuni also accompanied Chkhenkeli as a delegate to the Batumi Peace Conference that began on May 11, 1918. The conference saw the Ottomans extend their demands to include Akhaltsikhe and Akhalkalaki in Tiflis Governorate and the western half of Erivan Governorate. Before the Transcaucasus delegation had delivered a response, Ottoman forces invaded the Erivan Governorate, and on May 15 captured Alexandropol. A week later, they had approached both Yerevan and Karakilisa. Unable to negotiate anything more favorable than capitulation with the Ottomans, the Georgian leaders at the Batumi talks arranged a side-deal with Germany to exchange German protection for access to Georgia's economic resources. The result was that the Seym dissolved the federative republic on May 26, 1918, with the Democratic Republic of Georgia declared the same day and the republics of Azerbaijan and Armenia declared on May 28.

The Armenian National Council elected Kajaznuni as the first prime minister of the independent Armenian state on June 6, 1918 and his cabinet was formed on June 30. Kajaznuni held this position until August 7, 1919; with the nomination as a prime minister abroad from June 5, 1919. He was in diplomatic missions in Europe (beginning in August 1919) and the United States (from October 9, 1919 until August 1920). He returned to Armenia to become chairman of the parliament on November 4, 1920. Kajaznuni was arrested after the Bolsheviks came to power in December 1920 but was liberated during the February 1921 revolt against the Soviet regime.

After the end of the revolt in early April 1921, he left the country and lived in Bucharest from 1921 to 1924. In 1925 he returned to Soviet Armenia and worked as an architect in Leninakan. He also taught at the technical department of Yerevan State University, lecturing on construction and architecture. In 1930 he joined the newly established Construction Institute and attained the title of professor there. Kajaznuni became a member of the Armenian Union of Architects. Kajaznuni was a victim of Stalin's Great Terror—arrested in 1937 and imprisoned, he died in prison in 1938. The exact date of his death is unknown.