Middle Welsh (Welsh: Cymraeg Canol, Middle Welsh: Kymraec) is the label attached to the Welsh language of the 12th to 15th centuries, of which much more remains than for any earlier period. This form of Welsh developed directly from Old Welsh (Welsh: Hen Gymraeg).

Middle Welsh is the language of nearly all surviving early manuscripts of the Mabinogion, although the tales themselves are certainly much older. It is also the language of most of the manuscripts of mediaeval Welsh law. Middle Welsh is reasonably intelligible, albeit with some work, to a modern-day Welsh speaker.

The phonology of Middle Welsh is quite similar to that of modern Welsh, with only a few differences. The letter u, which today represents /ɨ/ in North Western Welsh dialects and /i/ in South Welsh and North East Welsh dialects, represented the close central rounded vowel /ʉ/ in Middle Welsh. The diphthong aw is found in unstressed final syllables in Middle Welsh, while in Modern Welsh it has become o (e.g. Middle Welsh marchawc = Modern Welsh marchog "horseman"). Similarly, the Middle Welsh diphthongs ei and eu have become ai and au in final syllables, e. g. Middle Welsh seith = modern saith "seven", Middle Welsh heul = modern haul "sun".

The vowels are as follows:

Vowel length is predictable: vowels are long in monosyllables unless followed by a geminate or one of the consonants /p/, /t/, /k/, /m/, /ŋ/. The vowels could combine into the following falling diphthongs:

1. ending in /w/: /aw/, /ew/, /iw/, /ɨw/ ~ /əw/

2. ending in /ɨ/: /aɨ/, /oɨ/, /uɨ/

3. others: /ej/, /eʉ/ (and possibly /æj/, /æʉ/)