Isthmus Nahuatl (Isthmus Nahuat; native name: melaꞌtájto̱l) is a Nahuatl dialect cluster spoken by about 30,000 people in Veracruz, Mexico. According to Ethnologue 16, the Cosoleacaque dialect is 84% intelligible with Pajapan, and 83% intelligible with Mecayapan.

It is spoken in the following Veracruzan communities:

The following description is that of Mecayapan dialect.

H is used at the beginnings of words before u, and has no value of its own. C is used to represent /k/ before the vowels a and o, while qu is used before i and e.

A macron under the letter (a̱, e̱, i̱, o̱) is used to mark long vowels.

Stress on the second-last syllable of a word that does not end in l or r, and stress on the last syllable of a word that does end in l or r, is unmarked. All other stress patterns are marked with an acute accent on the stressed vowel (á, é, í, ó).

The letters f, k, v and z occur only in loanwords.

This variety of Nahuatl has developed a distinction between inclusive and exclusive "we", which Classical Nahuatl and other modern forms of Nahuatl lack. The exclusive form is regularly derived from the first person singular ("I"), while the inclusive continues the suppletive first person plural of Classical Nahuatl.