Yes, Serbs existed in the regions west of the Drina, Una, Bosna, Herzegovina, and along the Adriatic Sea before the 14th century. Their presence and influence in these areas are documented through various historical sources, though the exact nature and extent of their settlement and political control varied over time.

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The medieval period in Serbia began with Slavic migrations to the Balkans in the 6th century, and the first Serbian state with an established political identity was founded in the mid-9th century by Prince Vlastimir.[1] The Byzantine Emperor Constantine VII Porphyrogenitus's work, De Administrando Imperio (DAI), compiled between 948 and 952, is a crucial source for understanding the early Serbs and their dwelling places.[1]

The DAI mentions that Serbs, originating from "White Serbs" in a land called Bojka (believed to be around Bohemia and Saxony), migrated to the Balkans during the reign of Emperor Heraclius (610–641 AD).[1] While the DAI's account has been reassessed for anachronisms, it indicates an early Serbian presence in the region.[1]

Presence in regions west of the Drina, Una, and Bosna:

• Bosnia: The DAI explicitly mentions Bosnia (χωριον βοσονα, "small land of Bosnia") as a region of "Baptized Serbia."[1] This indicates a Serbian presence and political connection to Bosnia in the mid-10th century. However, the interpretation of this statement has been debated, with some scholars suggesting it reflects temporary Serbian rule rather than an inherent "Serb land."[2] Later, the 12th-century Byzantine historian John Kinnamos described the Drina river as dividing Bosnia from the rest of Serbia, noting that Bosnia was ruled separately but was considered a "Serbian region" by 1155.[1]
• Dalmatia (Roman sense): The Royal Frankish Annals of 822 recorded Serbs "who controlled great part of Dalmatia" ("ad Sorabos, quae natio magnam Dalmatiae partem obtinere dicitur").[1] While "Dalmatia" in this context referred to a broader Roman province stretching to the Sava and Drina rivers, not just the modern coastal region, it suggests a significant Serbian presence in areas that would include parts of modern-day Bosnia and Croatia west of the Drina.[1] However, the extent of this control and the precise location of these Serbs are subject to scholarly discussion, with some arguing that the term "Dalmatia" was used broadly by the Franks and didn't necessarily imply widespread Serbian settlement across the entire area.[1]
• River Valleys: For the first two centuries after their settlement (from the early 7th century), Serbs inhabited narrow valleys in the watersheds of rivers such as the Lim, Tara, Piva, Ibar, West Morava, Upper Drina, and Upper Bosna.[1] This directly places them in areas west of the Drina and along the Bosna river basin.

Presence in Herzegovina and along the Adriatic Sea:

• Herzegovina (Zachlumia and Travunija): The principalities of Zachlumia and Travunija, which largely correspond to modern Herzegovina, were under Serbian political influence.[1] The DAI states that the "archons of Travunija were always under the rule of archons of Serbia."[1] Michael Višević, a ruler of Zachlumia, was an ally of the Bulgarian emperor Simeon but later allied with the Byzantines, indicating a distinct, yet often intertwined, political entity with Serbian connections.[1]
• Adriatic Coast (Pagania/Narentines and Duklja):
  • Pagania (Narentines): This region, located between the Neretva and Cetina rivers, was mentioned as being subordinated to Serbia during the reign of Prince Petar Gojniković in the early 10th century.[1] The Narentines, a Slavic tribe, were not Christianized with the rest of the Serbian tribes, leading the Greeks to call them "Pagans."[1]
  • Duklja: This medieval Serb state encompassed territories of present-day southeastern Montenegro, from the Bay of Kotor to the Bojana river, and inland to the Zeta and Morača rivers.[1] It was first mentioned in 10th-century Byzantine chronicles, and its rulers, such as Stefan Vojislav and Mihailo Vojislavljević, were described as Serbs or rulers of Serbs.[1] Mihailo was even proclaimed the first Serbian king in 1076 or 1077 by Pope Gregory VII, being addressed as "King of the Slavs" (Michaeli Sclavorum regi).[1] This clearly indicates a Serbian polity along the Adriatic coast before the 14th century.

While the exact boundaries and the degree of direct control fluctuated, historical records from the 9th, 10th, 11th, and 12th centuries consistently point to the presence of Serbs and Serbian political entities in these western and coastal regions of the Balkans well before the 14th century.[1] The awareness of a shared state and ethnic identity between medieval Bosnia and Serbia, particularly among the nobility, also existed, though it diminished over time due to differing political and social developments.[2]