Viktor Amazaspovich Ambartsumian (18 September [O.S. 5 September] 1908 – 12 August 1996) was a Soviet Armenian astrophysicist and science administrator.

One of the 20th century's top astronomers, he is widely regarded as the founder of theoretical astrophysics in the Soviet Union.

Educated at Leningrad State University (LSU) and the Pulkovo Observatory, Ambartsumian taught at LSU and founded the Soviet Union's first department of astrophysics there in 1934. He subsequently moved to Soviet Armenia, where he founded the Byurakan Observatory in 1946. It became his institutional base for the decades to come and a major center of astronomical research. He also co-founded the Armenian Academy of Sciences and led it for almost half a century—the entire post-war period. One commentator noted that "science in Armenia was synonymous with the name Ambartsumian." In 1965 Ambartsumian founded the journal Astrofizika and served as its editor for over 20 years.

Ambartsumian began retiring from the various positions he held only from the age of 80. He died at his house in Byurakan and was buried on the grounds of the observatory. He was declared a National Hero of Armenia in 1994.

Ambartsumian was born in Tiflis on 18 September (5 September in Old Style), 1908 to Hripsime Khakhanian (1885–1972) and Hamazasp Hambardzumyan (1880–1966).Hripsime's father was an Armenian Apostolic priest from Tskhinvali, while Hamazasp hailed from Vardenis (Basargechar). His ancestors moved from Diyadin, what is now Turkey, to the southern shores of Lake Sevan in 1830, in the aftermath of the Russo-Turkish War. Hamazasp (Russified: Amazasp) was an educated man of letters who studied law at Saint Petersburg University. He was also a writer and translator and notably translated Homer's Iliad into Armenian from Classical Greek. In 1912 he co-founded the Caucasian Society of Armenian Writers, which lasted until 1921. Ambartsumian was the secretary, while Hovhannes Tumanyan, the famed poet, served as its president.

Ambartsumian's parents married in 1904. He had a brother, Levon, and sister, Gohar. His brother, a geophysics student, died at 23-24 while on an expedition in the Urals. Gohar (1907–1979) was a mathematician and Chair of Probability Theory and Mathematical Statistics at Yerevan State University towards the end of her life.

Ambartsumian carried out basic research in astronomy and cosmogony. His research covered astrophysics, theoretical physics and mathematical physics. Most of his research focused on physics of nebulae, star systems, and extragalactic astronomy. He is best known for having discovered stellar associations and predicted activity of galactic nuclei. In his later career, Ambartsumian held views in contradiction to the consequences of the general relativity, such as rejecting the existence of black holes.