The focus is on the ways of designating the water area of the Gulf of Finland in written sources, primarily Swedish and Low German ones, starting from the Middle Ages through mid-Sixteenth century, i.e. before the names sinus Finnicus and mare Finnonicum, which appeared in the 1530s European cartographical tradition and then resulted in the place name we are used to, became widespread. The evidence available suggests that, during the period in question, no commonly used place names existed for the Gulf of Finland. Mare Estonum from the so-called Danish itinerary (ca. 1300) has not been attested elsewhere. As for the expressions like Finksii potok (or protok), i.e. ‘the Finnish stream’, found in the 16th century Russian-Swedish diplomatic correspondence, they reflect the Swedish and Low German word usage and should be understood as meaning ‘Finland’s coastal waters’. The word potok almost surely renders the Swedish ström, which (as well as the Low German strom) can mean ‘stream’ and ‘sound’ (‘strait’) but these meanings were not appropriate in the given context. The expression morskaia proliva (‘sea sound’ or ‘sea strait’, probably the result of calquing the Swedish ström) in The Description of the Three Routes by Afanasii of Kholmogory (1701) cannot be considered as a name of the Gulf of Finland either. It is possible that conceptualization of the Gulf of Finland as a separate water area, worth of receiving a special name, had been preconditioned by the development of European cartography.