Old Spanish (roman, romançe, romaz; Spanish: español antiguo), also known as Old Castilian or Medieval Spanish, refers to the varieties of Ibero-Romance spoken predominantly in Castile and environs during the Middle Ages. The earliest, longest, and most famous literary composition in Old Spanish is the Cantar de mio Cid (c. 1140–1207).

(/s/ and /z/ were apico-alveolar.)

These were still distinct phonemes in Old Spanish, judging by the consistency with which the graphemes ⟨b⟩ and ⟨v⟩ were distinguished. Nevertheless, the two could be confused in consonant clusters (as in alba~alva “dawn”) or in word-initial position, perhaps after /n/ or a pause. /b/ and /β/ appear to have merged in word-initial position by about 1400 and in all other environments by the mid–late 16th century at the latest.

At an archaic stage, the realizations of /h/ (from Latin /f/) would have been approximately as follows:

By early Old Spanish, [ɸ] had been replaced with [h] before all vowels and possibly before [j] as well.

In later Old Spanish, surviving [ɸ] and [ʍ]/[hɸ] were modified to [f] in urban speech, likely due to the influx of numerous French and Occitan speakers (and their particular pronunciation of Latin) beginning in the twelfth century. Various words with [f] were then borrowed into Spanish, leading to minimal pairs like [ˈfoɾma] “form” (a borrowing) and [ˈhoɾma] “shoemaker's last” (inherited from Latin forma). The result was a new phoneme /f/, distinct from /h/.

Possibly realized as [d͡ʒ] after pauses or certain consonants (judging by outcomes in Judeo-Spanish).

Old Spanish was generally written in some variation of the Latin script. It was also sometimes written in Arabic script in a practice called Aljamía.