At the time of the Roman Empire, the Baltic Sea was known as the Mare Suebicum or Mare Sarmaticum. Tacitus in his AD 98 Agricola and Germania described the Mare Suebicum, named for the Suebi tribe, during the spring months, as a brackish sea when the ice on the Baltic Sea broke apart and chunks floated about. The Suebi eventually migrated south west to reside for a while in the Rhineland area of modern Germany, where their name survives in the historic region known as Swabia. The Sarmatian tribes inhabited Eastern Europe and southern Russia. Jordanes called it the Germanic Sea in his work the Getica.
Since the Viking age, the Scandinavians have called it „the Eastern Lake“ (Austmarr, „Eastern Sea“, appears in the Heimskringla and Eystra salt appears in Sörla þáttr), but Saxo Grammaticus recorded in Gesta Danorum an older name Gandvik, „-vik“ being Old Norse for „bay“, which implies that the Vikings correctly regarded it as an inlet of the sea. (Another form of the name, „Grandvik“, attested in at least one English translation of Gesta Danorum, is likely to be a misspelling.)
In addition to fish the sea also provides amber, especially from its southern shores. The bordering countries have traditionally provided lumber, wood tar, flax, hemp, and furs. Sweden had from early medieval times also a flourishing mining industry, especially on iron ore and silver. Poland had and still has extensive salt mines. All this has provided for rich trading since the Roman times.
In the early Middle Ages, Vikings of Scandinavia fought for control over the sea with Slavic Pomeranians. The Vikings used the rivers of Russia for trade routes, finding their way eventually to the Black Sea and southern Russia.
Lands next to the sea’s eastern shore were among the last in Europe to be converted into Christianity in the Northern Crusades: Finland in the 12th century by the Swedes, and what are now Estonia and Latvia in the early 13th century by the Danes and the Germans (Livonian Brothers of the Sword). The powerful German Teutonic Knights gained control over most of the southern and eastern shore of the Baltic Sea, while fighting the Poles, the Danes, the Swedes, the Russians of ancient Novgorod, and the Lithuanians (latest of all Europeans to convert to Christianity).