The Azerbaijanis or Azeris are a Turkic ethnic group of mixed ethnic origins, primarily the indigenous peoples of eastern Transcaucasia, the Medians, an ancient Iranian people, and the Oghuz Turkic tribes that began migrating to Azerbaijan in the 11th century CE. Modern Azerbaijanis are the second most numerous ethnic group among the Turkic peoples after Anatolian Turks and speak North Azerbaijani (spoken mainly in the Republic of Azerbaijan and Russia) and/or South Azerbaijani (spoken in Iran, Iraq, and Syria). Both languages also have dialects, with 21 North Azerbaijani dialects and 11 South Azerbaijani dialects.

The Azerbaijani are of mixed ethnic origin, the oldest element deriving from the indigenous population of eastern Transcaucasia and possibly from the Medians of northern Persia. Between the 3rd and 7th centuries CE, there was a Persianization of the population, during the period of the Sassanian dynasty. A subsequent Turkification of the population occurred after the region's conquest by the Seljuq Turks in the 11th century and a continued influx of Turkic peoples over subsequent centuries – including groups that migrated during the Mongol conquests of the 13th century (most of the tribes that formed the Mongol forces, or were led by them to migrate were Turkic peoples.)

The Caucasian origin of the Azerbaijanis defines a link between Azerbaijanis and their pre-Turkification Caucasian past and mostly applies to the Azerbaijanis of the Caucasus, most of whom are now inhabitants of what is now the Republic of Azerbaijan. There is evidence that, due to repeated invasions and migrations, aboriginal Caucasians were culturally assimilated, first by Iranians, such as the Alans, and later by the Oghuz Turks. Considerable information has been learned about the Caucasian Albanians including their language, history, early conversion to Christianity, and close ties to the Armenians. Many academics believe that the Udi language, still spoken in Azerbaijan, is a remnant of the Albanians' language.

This Caucasian influence extended further south into Iranian Azerbaijan. During the 1st millennium BCE, another Caucasian people, the Mannaeans (Mannai) populated much of this area. This ancient country was in northwestern Iran, south of Lake Urmia. During the period of its existence in the early 1st millennium BCE, Mannai was surrounded by three major powers: Assyria, Urartu, and Media. With the intrusion of the Scythians and the rise of the Medes in the 7th century, the Manneans lost their identity and were subsumed under the term Medes.

Although genetic testing demonstrates that the genetic heritage of Azerbaijani Turks is mostly from the native populations of the Middle East and the Caucasus, rather than their being direct descendants of migrants from Central Asia, it does, however, show that the region is a genetically mixed one. Whereas the population of Azerbaijan is culturally diverse, genetic testing has revealed common genetic markers that support an autochthonous (indigenous) background for most Azerbaijani Turks. MtDNA analysis indicates that the main relationship with Iranians is through a larger West Eurasian group that is secondary to that of the Caucasus, according to a study that did not include Azerbaijani Turks, but Georgians who have clustered with Azerbaijani Turks in other studies. The conclusion from the testing shows that Azerbaijani Turks of the republic are a mixed population with relationships, in order of greatest similarity, with the Caucasus, Iranians and Near Easterners, Europeans, and Turkmen. Other genetic analysis of mtDNA (maternal) and Y-chromosomes (paternal) indicates that Caucasian populations are genetically intermediate between Europeans and Near Easterners, but that they are more closely related to Near Easterners overall. Another study, conducted in 2003 by the Russian Journal of Genetics, compared Iranian-language speakers of the Republic of Azerbaijan (the Talysh and Tats) with Turkic Azerbaijanis and found that the genetic structure of those populations, compared with the other Iranian-speaking populations (Persians from Iran, Ossetins, and Tajiks), was closer to Turkic Azerbaijanis than to Iranian-speaking populations elsewhere.

In 2006, M. Regueiro and A.M. Cadenas of Stanford University showed that the population of central Iran (Isfahan) was closer to Caucasian Azerbaijani people than to the population of Turkey in terms of haplogroup distributions and genetic homogeneity.

The latest comparative study (2013) on the complete mitochondrial DNA diversity in Iranians has indicated that Iranian Azerbaijanis are more related to the people of Georgia than they are to other Iranians and to Armenians. However, the same multidimensional scaling plot shows that Azerbaijanis from the Caucasus, despite their supposed common origin with Iranian Azerbaijanis, cluster closer with other Iranians (e.g. Persians, etc.) than they do with Iranian Azerbaijanis.

Azerbaijani Turks, however, are still demonstrated to possess significant genetic influences from East Asia relative to their non-Turkic neighbors.