Shtokavian or Štokavian (/ʃtɒˈkɑːviən, -ˈkæv-/; Serbo-Croatian Latin: štokavski / Serbo-Croatian Cyrillic: штокавски, pronounced [ʃtǒːkaʋskiː]) is the prestige supradialect of the pluricentric Serbo-Croatian language and the basis of its Serbian, Croatian, Bosnian and Montenegrin standards. It is a part of the South Slavic dialect continuum. Its name comes from the form for the interrogative pronoun for "what": što. This is in contrast to Kajkavian and Chakavian (kaj and ča also meaning "what").

Shtokavian is spoken in Serbia, Montenegro, Bosnia and Herzegovina, much of Croatia, and the southern part of Austria's Burgenland. The primary subdivisions of Shtokavian are based on three principles: one is different accents (whether the subdialect is Old-Shtokavian or Neo-Shtokavian), second is the way the old Slavic phoneme yat has changed (Ikavian, Ijekavian or Ekavian), and third is the presence of the Young Proto-Slavic isogloss (Schakavian or Shtakavian). Modern dialectology generally recognises seven Shtokavian subdialects.

The early medieval Slavs who later spoke various Bulgarian and Serbo-Croatian dialects migrated across Moldavia and Pannonia. According to Frederik Kortlandt, the shared innovations originate from a "Trans-Carpathian" homeland, and by the 4th and 6th century, "the major dialect divisions of Slavic were already established". Dialectologists and Slavists maintain that when the separation of Western South Slavic speeches happened, they separated into five divergent groups, more specifically two, one Slovene and a second Serbo-Croatian with four divergent groups - Kajkavian, Chakavian, Western Shtokavian and Eastern Shtokavian. The latter group can be additionally divided into a first (Kajkavian, Chakavian, Western Shtokavian) and second (Eastern Shtokavian, Torlakian). As noted by Ranko Matasović, "the Shtokavian dialect, on the other hand, was from the earliest times very non-unique, with the Western Shtokavian dialects leaning towards Kajkavian, and the Eastern Shtokavian to Torlakian". According to isoglosses, and presumed end of existence of the common Southwestern Slavic language around the 8th-9th century, the formation of the Proto-Western Shtokavian and Proto-Eastern Shtokavian linguistic and territorial unit would be around the 9th-10th century (Proto-Western Shtokavian closer to Proto-Chakavian, while Proto-Eastern Shtokavian shared an old isogloss with Bulgarian). According to Ivo Banac in the area of today's Slavonia, Bosnia and Herzegovina (west of Brčko, Vlasenica and Neretva line) and on the littoral between the Bay of Kotor and Cetina, medieval Croats spoke an old West Shtokavian dialect, which, some believe, stemmed from Chakavian, while medieval Serbs spoke two dialects, old East Shtokavian and Torlakian. Many linguists noted a close connection between Chakavian and Western Shtokavian, for example Pavle Ivić saw Chakavian as an archaic peripheral zone of Shtokavian, while Dalibor Brozović saw the majority of Chakavian dialects as derived from the same accentological core as Western Shtokavian. Western Shtokavian was principally characterized by a three-accent system, whereas Eastern Shtokavian was mostly marked by a two-accent system.

Western Shtokavian covered the major part of present-day Bosnia and Herzegovina, Slavonia and part of Southern Dalmatia in Croatia. Eastern Shtokavian was dominant in Serbia, easternmost Bosnia and Herzegovina and greater parts of Montenegro. From the 12th century, both dialects started separating further from Chakavian and Kajkavian idioms. According to research of historical linguistics, Old-Shtokavian was well established by the mid-15th century. In this period it was still mixed with Church Slavonic to varying degrees. However, the ultimate development of Western Shtokavian and Eastern Shtokavian was not divergent (like in the case of Chakavian and Kajkavian), but convergent. It was the result of migrations (particularly of Neoshtokavian-Eastern Shtokavian speakers), political-cultural border change and also caused by the Ottoman invasion (since the 16th century). Initially two separate proto-idioms started to resemble each other so greatly that, according to Brozović (1975), "[today] we can no longer speak of an independent Western Shtokavian, but only about the better or weaker preservation of former West Shtokavian features in some dialects of the unique Shtokavian group of dialects".

As can be seen from the image on the right, originally the Shtokavian dialect covered a significantly smaller area than it covers today, meaning that the Shtokavian speech has spread for the last five centuries, overwhelmingly at the expense of Chakavian and Kajkavian idioms. The modern areal distribution of these three dialects as well as their internal stratification (Shtokavian and Chakavian in particular) is primarily a result of the migrations resulting from the spread of the Ottoman Empire in the Balkans. Migratory waves were particularly strong in the 16th–18th century, bringing about large-scale linguistic and ethnic changes in the Central South Slavic area (see also Great Serb Migrations).

By far the most numerous, mobile and expansionist migrations were those of Ijekavian-Shtokavian speakers of eastern Herzegovina, who have spread into most of Western Serbia, many areas of eastern and western Bosnia, large swathes of Croatia (Banovina, Kordun, Lika, parts of Gorski kotar, continental parts of northern Dalmatia, some places north of Kupa, parts of Slavonia, southeastern Baranya etc.). This is the reason Eastern Herzegovinian is the most spoken Serbo-Croatian dialect today, and why its name is only descriptive of its area of origin. These migrations also played a pivotal role in the spread of Neo-Shtokavian innovations.

Proto-Shtokavian, or Church Slavic with elements of nascent Shtokavian, were recorded in legal documents like the charter of Ban Kulin, regulating the commerce between Bosnia and Dubrovnik in Croatia, dated 1189, and in liturgical texts like Gršković's and Mihanović's fragments, c. 1150, in southern Bosnia or Herzegovina. Experts' opinions are divided with regard to the extent these texts, especially the Kulin ban parchment, contain contemporary Shtokavian vernacular. Numerous legal and commercial documents from pre-Ottoman Bosnia, Hum, Serbia, Zeta, and southern Dalmatia, especially Dubrovnik are mainly Shtokavian, with elements of Church Slavic. The first major comprehensive vernacular Shtokavian text is the Vatican Croatian Prayer Book, written in Dubrovnik a decade or two before 1400. In the next two centuries Shtokavian vernacular texts had been written mainly in Dubrovnik, other Adriatic cities and islands influenced by Dubrovnik, as well as in Bosnia, by Bosnian Franciscans and Bosnian Muslim vernacular aljamiado literature – the first example being "Chirvat-türkisi" or "Croatian song", dated 1589.

Shtokavian is characterized by a number of characteristic historical sound changes, accentual changes, changes in inflection, morphology and syntax. Some of these isoglosses are not exclusive and have also been shared by neighboring dialects, and some of them have mostly but not completely spread over the whole Shtokavian area. The differences between Shtokavian and the unrelated, neighboring Bulgarian–Macedonian dialects are mostly clear-cut, whereas the differences with the related Serbo-Croatian dialects of Chakavian and Kajkavian are much more fluid, and the mutual influence of various subdialects plays a more prominent role.