Duklja (Serbian Cyrillic: Дукља; Greek: Διόκλεια, romanized: Diokleia; Latin: Dioclea) was a medieval South Slavic state which roughly encompassed the territories of modern-day southeastern Montenegro, from the Bay of Kotor in the west to the Bojana river in the east, and to the sources of the Zeta and Morača rivers in the north. First mentioned in 10th– and 11th-century Byzantine chronicles, it was a vassal of the Bulgarian Empire between 997 and 1018, and then of the Byzantine Empire until it became independent in 1040 under Stefan Vojislav (fl. 1034–43) who rose up and managed to take over territories of the earlier Serbian Principality, founding the Vojislavljević dynasty. Between 1043 and 1080, under Mihailo Vojislavljević (r.  1050–81), and his son, Constantine Bodin (r.  1081–1101), Duklja saw its apogee. Mihailo was given the nominal title King of Slavs by the Pope after having left the Byzantine camp and supported an uprising in the Balkans, in which his son Bodin played a central part. Having incorporated the Serbian hinterland (known as Grand Principality of Serbia, and anachronistically as Raška) and installed vassal rulers there, this maritime principality emerged as the most powerful Serb polity, seen in the titles used by its rulers ("Prince of Serbia", "of Serbs"). However, its rise was short-lived, as Bodin was defeated by the Byzantines and imprisoned; pushed to the background, his relative and vassal Vukan became independent in Raška, which continued the fight against the Byzantines while Duklja was struck with civil wars. Between 1113 and 1149 Duklja was the centre of Serbian–Byzantine conflict, with members of the Vojislavljević as protégés of either fighting each other for power. Duklja was then incorporated as a crown land of the Grand Principality of Serbia ruled by the Vukanović dynasty, subsequently known as Zeta, remaining so until the fall of the Serbian Empire in the 14th century.

In historiography, K. Jirechek was the first to use "Duklja". Doclea was originally the name of the Roman city on the site of modern Podgorica (Ribnica), built by Roman Emperor Diocletian, who hailed from this region of Roman Dalmatia. The Romanized Illyrian tribe known as Docleatae that inhabited the area derived their name from the city. In later centuries, the Romans hypercorrected the name to Dioclea, wrongly guessing that an i had been lost due to vulgar speech patterns. Duklja is the later Slavic version of the name of this region, attributed to the principality under Byzantine suzerainty ("Diokleia", Διόκλεια). The demonym, or tribal name, appearing in De Administrando Imperio was "Dioklētianoi" (Διοκλητιανοί).

According to De Administrando Imperio (948–952), in chapter 35, Diokleia (Διόκλεια) included the "large, inhabited cities" of Gradetai, Nougrade, and Lontodokla. Gradetai may have been Starigrad, Nougrade may have been Prevlaka, while the location of Lontodokla is uncertain.

According to the later, somewhat dubious source, Chronicle of the Priest of Duklja, a ruler named Hvalimir who was alleged to be an ancestor of Jovan Vladimir (ca. 990–1016), held Zeta and its towns, and the following counties: Lusca (Luška), Podlugiae (Podlužje), Gorsca (Gorska), Cupelnich (Kupelnik), Obliquus (Oblik), Prapratna, Cermeniza (Crmnica) and Budua (Budva) with Cuceva (Kučevo) and Gripuli (Grbalj).

Since the 12th century, the term Zeta began to replace the name Duklja.

The DAI claims that Duklja had been made desolate by the Avars and "repopulated in the time of the Emperor Heraclius, just as were Croatia and Serbia" (i.e. in the first half of the 7th century), by Slavs. While he clearly states that the neighbouring principalities of Serbia, Pagania, Zahumlje, Travunia and Kanalites had been settled by the 'unbaptized Serbs', he mentions Duklja simply as having been settled by 'Slavs'. The statements of various Byzantine writers in which the Diocleans are also called as Serbs, Croats, and Dalmati do not allow equalization of Duklja inhabitants until 12th century either with the Serbs or with the Croats. Scholars have debated at length as to the reliability of DAI. For example, Francis Dvornik and Florin Curta, among others, suggested that the DAI was a political document, rather than a strictly historical one and that it probably indicates that the coastal županijas were under the authority of the Serbian prince, Časlav Klonimirović, in the mid-10th century. Tibor Živković, Neven Budak and Hrvoje Gračanin also concluded that a closer reading suggests that Constantine consideration about regional ethnic identity is based on Serbian political rule and expansion in the 10th century which does not necessarily indicate ethnic origin. Relja Novaković also came to a similar conclusion. John V.A. Fine argues "given that Serbs settled in regions along its borders, presumably this would have also been a Serb region". Ivo Banac proposed that the DAI mention that a part of the Croats "split off and took control of Illyricum and Pannonia" after settling western part of the province of Dalmatia could be connected to Duklja, while Chronicle of the Priest of Duklja calls that area as Red Croatia. According to historian Sima Ćirković, although Constantine VII does not mention tribes of whom peoples of Duklja originate, the authors of the XI century considered rulers of Zeta to be Serbs and sometimes call their land Serbia. Ćirković also considers that this narrow scientific question has been politicized by opposing Diocleans to Serbs and ignoring historical sources from 11th and 12th century. The dubious Chronicle of the Priest of Duklja, compiled in 1298–1301 by a Cistercian monk in the service of Paul I Šubić of Bribir, nevertheless if does again promote mere Byzantine confusion over Serbs and Croats, with other historical sources does allude to the existence of Croats and their political influence far into Montenegrin inland until the late 12th century.

Ultimately, the origins of Duklja are not known with certainty, for the literary evidence often rests on semi-legendary genealogies. Moreover, what actually constituted a people (gens) in the Middle Ages has been rigorously debated. There is no clear evidence that peoples known as Serbs or Croats migrated en masse as coherent nations able to resettle large territories, and that arrived as small military elites which assimilated and organized other already settled Slavs. Both Florin Curta and John Fine, among other medievalists, have argued that ethnonyms such as Serb or Croat were primarily political labels referring to a dux and his retinue of nobles as well as military elite, while on a lower level it also referred the mass of commoners who inhabited the territory under the often nominal and transient rule of such leaders. There is little evidence that a modern notion of nation-type ethnicity, and the values associated with it, existed in medieval societies. Rather, some sort of group identity began to form within the Balkans from the late 7th century as Slavic notables formed a system of alliances. This coincides with the final demise of Avar hegemony over the western Balkans. At the same time, the Byzantines had begun to re-establish some control in parts of the Balkans after the 7th-century collapse of imperial control. The establishment of the Byzantine theme of Dyrrhachium facilitated diplomatic contacts between the East Romans and the Adriatic Sclaviniae.^{[citation needed]}

Rather, for the general masses, identity was rooted primarily with one's own clan, village and region. As Fine states, "In this large region settled by Slavs, all of whom spoke the same language, certain political entities emerged, and that is all that they were, political entities". Duklja was one such polity, and its subsequent history was closely intertwined with that of Serbia/Rascia and the Byzantine Empire, and as well as Rome and 'western' powers. As such Duklja is seen as one of the medieval Serb states and was the political and cultural predecessor of modern Montenegro.