Old Swedish (Modern Swedish: fornsvenska) is the name for two distinct stages of the Swedish language that were spoken in the Middle Ages: Early Old Swedish (klassisk fornsvenska), spoken from about 1225 until about 1375, and Late Old Swedish (yngre fornsvenska), spoken from about 1375 until about 1526.

Old Swedish developed from Old East Norse, the eastern dialect of Old Norse. The earliest forms of the Swedish and Danish languages, spoken between the years 800 and 1100, were dialects of Old East Norse and are referred to as Runic Swedish and Runic Danish because at the time all texts were written in the runic alphabet. The differences were only minute, however, and the dialects truly began to diverge around the 12th century, becoming Old Swedish and Old Danish in the 13th century. It is not known when exactly Elfdalian began to diverge from Swedish.

Early Old Swedish was markedly different from modern Swedish in that it had a more complex case structure and had not yet experienced a reduction of the gender system and thus had three genders. Nouns, adjectives, pronouns and certain numerals were inflected in four cases: nominative, genitive, dative and accusative.

The writing of the Westrogothic law marked the beginning of Early Old Swedish (klassisk fornsvenska or äldre fornsvenska; 1225–1375), which had developed from Old East Norse. It was the first Swedish language document written in the Latin alphabet, and its oldest fragments have been dated to around the year 1225.

Old Swedish was relatively stable during this period. The phonological and grammatical systems inherited from Old Norse were relatively well preserved and did not experience any major changes.

Most of the texts from the Early Old Swedish period were written in Latin, as it was the language of knowledge and the Church. However, Old Swedish was used as a literary language as well, and laws especially were written in it; of the 28 surviving manuscripts from this period, 24 contain law texts. Much of the knowledge of Old Swedish comes from these law texts. In addition to laws, some religious and poetic texts were also written in Old Swedish.

The Catholic Church and its various monastic orders introduced many new Greek and Latin loanwords into Old Swedish. Latin especially had an influence on the written language.

The Middle Low German language also influenced Old Swedish due to the economic and political power of the Hanseatic League during the 13th and 14th centuries. Accordingly, loanwords relating to warfare, trade, crafts and bureaucracy entered the Swedish language directly from Low German, along with some grammatical suffixes and conjunctions. The prefixes be-, ge- and för- that can be found in the beginning of modern Swedish words came from the Low German be-, ge- and vor-. Some words were replaced with new ones: the native word for window, vindøgha, was replaced with fönster, eldhus (kitchen) was replaced with kök and gælda (to pay) with betala. Some of these words still exist in Modern Swedish but are often considered archaic or dialectal; one example is the word vindöga (window). Many words related to seafaring were borrowed from Dutch.