Consonant gradation is a type of consonant mutation (mostly lenition but also assimilation) found in some Uralic languages, more specifically in the Finnic, Samic and Samoyedic branches. It originally arose as an allophonic alternation between open and closed syllables, but has become grammaticalised due to changes in the syllable structure of the languages affected.

The term "consonant gradation" refers to a word-medial alternation of consonants between fortis and lenis realisations. The fortis strong grade appears in historically open syllables (ending in a vowel), while the lenis weak grade appears in historically closed syllables (ending in a consonant). The exact realisation of the fortis–lenis distinction differs between the branches. In the Samic languages it was realised through fortition, specifically lengthening, in the strong grade. In the Finnic and Samoyedic languages, there was instead lenition in the weak grade. Thus, the exact realization of the contrast is not crucial.

The language groups differ in regard to their treatment sequences of a vowel followed by j or w in Proto-Uralic. In the Samic languages, the second part of these remains phonologically a consonant, and can thus close the syllable before it, triggering the weak grade. It also takes part in gradation itself, lengthening in the strong grade. In Finnic, on the other hand, these were treated as diphthongs, and were equivalent to long vowels in terms of syllable structure. Consequently, they did not close the syllable and did not affect gradation.

Consonant gradation is understood to have originally been a predictable phonological process. In all languages that retain it, however, it has evolved further to a less predictable system of consonant mutation, of morphophonological or even purely morphological nature. This is a consequence of later changes in the structure of syllables, which made closed syllables open or vice versa, without adjusting the gradation. For example, in Northern Sami, the only difference between giella and giela ('language', nominative and genitive singular respectively) is the grade; the final consonant that originally closed the syllable in the genitive form, namely -n, has disappeared (the same happened in the history of Estonian and Nganasan). Even in Finnish, which is relatively conservative with respect to consonants, there are many cases of strong grades in closed syllables and weak grades in open syllables, e.g. sade and sateen ('rain', nominative and genitive singular). These, again, are the result of changes in syllable structure, with the original Proto-Finnic *sadek and *sategen following the rules more obviously. In addition, not all Finnish words that hypothetically could have gradation undergo it: for instance, the genitive form of auto 'car' is auton. Thus, the occurrence of gradation is not morphologically predictable anymore, it is a property of each individual word.

There is no consensus view on the ultimate origin of consonant gradation in the Uralic languages. Three broad positions may be distinguished:

In all three groups, consonant gradation has the same conditioning, the distinction between open and closed syllables. In this light, and in the absence of any evidence of the same system having existed in any unrelated language in the world, Helimski (1995) has argued that the latter two options should be rejected as implausible.

If a connection exists, it is also disputed what its nature may be, again allowing for three broad positions:

The great geographical distance between the Finnic and Samic peoples on one hand, and the Nganasans on the other, leads Helimski to reject the second option of these.