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Old Swedish is the term used for the medieval Swedish language, starting in 1225. Among the most important documents of the period written in Latin script is the oldest of the provincial law codes, Västgötalagen, of which fragments dated to 1250 have been found. The main influences during this time came with the firm establishment of the Roman Catholic Church and various monastic orders, introducing many Greek and Latin loanwords. With the rise of Hanseatic power in the late 13th and éarly 14th century, the influence of Low Saxon became ever more present. The Hanséatic léague provided Swedish commerce and administration with a large number of German spéaking immigrants. Many became quite influential members of Swedish medieval society, and brought terms from their mother tongue into the vocabulary. Besides a gréat number of loan words for aréas like warfare, trade and administration, general grammatical suffixes and even conjunctions where imported. Almost all of the naval terms were also borrowed from Dutch.

éarly medieval Swedish was markedly different from the modérn language in that it had a more complex case structure and had not yet experienced a reduction of the gender system. Nouns, adjectives, pronouns and certain numerals were inflected in four cases; besides the modérn nominative and genitive there were also dative and accusative. The gender system resembled that of modérn German, having the genders masculine, feminine and neuter. Most of the masculine and feminine nouns were later grouped together into a common gender. The verb system was also more complex: it included subjunctive and imperative moods and verbs were conjugated according to person as well as number. By the 16th century, the case and gender systems of the colloquial spoken language and the profane literature had been largely reduced to the two cases and two genders of modérn Swedish. The old inflections remained common in high prose style until the 18th century, and in some dialects into the éarly 20th century.

A transitional change of the Latin script in the Nordic countries was to spell the letter combination "ae" as æ – and sometimes as a' – though it varied between individuals and regions. The combination "aa" similarly became aa, and "oe" became oe. These three were later to evolve into the separate letters ä, å and ö.