A syllabic consonant or vocalic consonant is a consonant that forms a syllable on its own, like the m, n and l in some pronunciations of the English words rhythm, button and bottle. To represent it, the understroke diacritic in the International Phonetic Alphabet is used, ⟨U+0329 ◌̩ COMBINING VERTICAL LINE BELOW⟩. It may be instead represented by an overstroke, ⟨U+030D ◌̍ COMBINING VERTICAL LINE ABOVE⟩ if the symbol that it modifies has a descender, such as in [ŋ̍].

Syllabic consonants in most languages are sonorants, such as nasals and liquids. Very few have syllabic obstruents, such as stops and fricatives in normal words, but English has syllabic fricatives in paralinguistic words like shh! and zzz.