Azerbaijanis (/ˌæzərbaɪˈdʒæni, -ɑːni/; Azerbaijani: Azərbaycanlılar, آذربایجانلیلار), Azeris or Azerbaijani Turks (Azərbaycan türkləri, آذربایجان تۆرکلری) are a Turkic ethnic group living mainly in the Azerbaijan region of northwestern Iran and the Republic of Azerbaijan. They are predominantly Shia Muslims. They comprise the largest ethnic group in the Republic of Azerbaijan and the second-largest ethnic group in neighboring Iran and Georgia. They speak the Azerbaijani language, belonging to the Oghuz branch of the Turkic languages.

Following the Russo-Persian Wars of 1813 and 1828, the territories of Qajar Iran in the Caucasus were ceded to the Russian Empire and the treaties of Gulistan in 1813 and Turkmenchay in 1828 finalized the borders between Russia and Iran. After more than 80 years of being under the Russian Empire in the Caucasus, the Azerbaijan Democratic Republic was established in 1918 which defined the territory of the Republic of Azerbaijan.

Azerbaijan is believed to be named after Atropates, a Persian satrap (governor) who ruled in Atropatene (modern Iranian Azerbaijan) circa 321 BC. The name Atropates is the Hellenistic form of Old Persian Aturpat which means 'guardian of fire' itself a compound of ātūr () 'fire' (later āður (آذر) in (early) New Persian, and is pronounced āzar today) + -pat () suffix for -guardian, -lord, -master (-pat in early Middle Persian, -bod (بُد) in New Persian).

Present-day name Azerbaijan is the Arabicized form of Āzarpāyegān (Persian: آذرپایگان) meaning 'the guardians of fire' later becoming Azerbaijan (Persian: آذربایجان) due to the phonemic shift from /p/ to /b/ and /g/ to /dʒ/ which is a result of the medieval Arabic influences that followed the Arab invasion of Iran, and is due to the lack of the phoneme /p/ and /g/ in the Arabic language. The word Azarpāyegān itself is ultimately from Old Persian Āturpātakān (Persian: آتورپاتکان) meaning 'the land associated with (satrap) Aturpat' or 'the land of fire guardians' (-an, in its postvocalic form -kān, is a suffix for association or forming adverbs and plurals; e.g.: Gilan 'land associated with Gil people').

The modern ethnonym "Azerbaijani" or "Azeri" refers to the Turkic peoples of Iran's northwestern historic region of Azerbaijan (also known as Iranian Azerbaijan) and the Republic of Azerbaijan. They historically called themselves or were referred to by others as Muslims and/or Turks. They were also referred to as Ajam (meaning from Iran), using the term incorrectly to denote their Shia belief rather than ethnic identity. When the Southern Caucasus became part of the Russian Empire in the nineteenth century, the Russian authorities, who traditionally referred to all Turkic people as Tatars, defined Tatars living in the Transcaucasus region as Caucasian Tatars or more rarely Aderbeijanskie (Адербейджанские) Tatars or even Persian Tatars in order to distinguish them from other Turkic groups and the Persian speakers of Iran. The Russian Brockhaus and Efron Encyclopedic Dictionary, written in the 1890s, also referred to Tatars in Azerbaijan as Aderbeijans (адербейджаны), but noted that the term had not been widely adopted. This ethnonym was also used by Joseph Deniker in 1900. In Azerbaijani language publications, the expression "Azerbaijani nation" referring to those who were known as Tatars of the Caucasus first appeared in the newspaper Kashkul in 1880.

During the early Soviet period, the term "Transcaucasian Tatars" was supplanted by "Azerbaijani Turks" and ultimately "Azerbaijanis." For some time afterwards, the term "Azerbaijanis" was then applied to all Turkic-speaking Muslims in Transcaucasia, from the Meskhetian Turks in southwestern Georgia, to the Terekemes of southern Dagestan, as well as assimilated Tats and Talysh. The temporary designation of Meskhetian Turks as "Azerbaijanis" was most likely related to the existing administrative framework of the Transcaucasian SFSR, as the Azerbaijan SSR was one of its founding members. After the establishment of the Azerbaijan SSR, on the order of Soviet leader Stalin, the "name of the formal language" of the Azerbaijan SSR was also "changed from Turkic to Azerbaijani".

The Chechen and Ingush names for Azerbaijanis are Ghezloy/Ghoazloy (ГӀезлой/ГӀоазлой) and Ghazaroy/Ghazharey (ГӀажарой/ГӀажарей). The former goes back to the name of Qizilbash while the latter goes back to the name of Qajars, having presumably emerged in Chechen and Ingush languages during the reign of Qajars in Iran in the 18th–19th centuries.

Ancient residents of the area, known as Azaris, spoke Old Azeri from the Iranian branch of the Indo-European languages. In the 11th century AD with Seljuq conquests, Oghuz Turkic tribes started moving across the Iranian Plateau into the Caucasus and Anatolia. The influx of the Oghuz and other Turkmen tribes was further accentuated by the Mongol invasion. These Turkmen tribes spread as smaller groups, a number of which settled down in the Caucasus and Iran, resulting in the Turkification of the local population. Over time they converted to Shia Islam and gradually absorbed Azerbaijan and Shirvan.