In linguistics, a consonant cluster, consonant sequence or consonant compound is a group of consonants which have no intervening vowel. In English, for example, the groups /spl/ and /ts/ are consonant clusters in the word splits. In the education field it is variously called a consonant cluster or a consonant blend.

Some linguists^[who?] argue that the term can be properly applied only to those consonant clusters that occur within one syllable. Others claim that the concept is more useful when it includes consonant sequences across syllable boundaries. According to the former definition, the longest consonant clusters in the word extra would be /ks/ and /tr/, whereas the latter allows /kstr/, which is phonetically [kst̠ɹ̠̊˔ʷ] in some accents.

Each language has an associated set of phonotactic constraints. Languages' phonotactics differ as to what consonant clusters they permit. Many languages are more restrictive than English in terms of consonant clusters, and some forbid consonant clusters entirely.

For example, Hawaiian, like most Oceanic languages, forbids consonant clusters entirely. Japanese is almost as strict, but allows a sequence of a nasal consonant plus another consonant, as in Honshū [hoꜜɰ̃ɕɯː] (the name of the largest island of Japan). It also permits geminate /kk/, /pp/, /ss/, and /tt/. However, palatalized consonants, such as [kʲ] in Tōkyō [toːkʲoː], are single consonants.

Standard Arabic forbids initial consonant clusters and more than two consecutive consonants in other positions, as do most other Semitic languages, although Modern Israeli Hebrew permits initial two-consonant clusters (e.g. pkak "cap"; dlaat "pumpkin"), and Moroccan Arabic, under Berber influence, allows strings of several consonants.

Like most Mon–Khmer languages, Khmer permits only initial consonant clusters with up to three consonants in a row per syllable. Finnish has initial consonant clusters natively only on South-Western dialects and on foreign loans, and only clusters of three inside the word are allowed. Most spoken languages and dialects, however, are more permissive. In Burmese, consonant clusters of only up to three consonants (the initial and two medials—two written forms of /-j-/, /-w-/) at the initial onset are allowed in writing and only two (the initial and one medial) are pronounced; these clusters are restricted to certain letters. Some Burmese dialects allow for clusters of up to four consonants (with the addition of the /-l-/ medial, which can combine with the above-mentioned medials).

At the other end of the scale, the Kartvelian languages of Georgia are drastically more permissive of consonant clustering. Clusters in Georgian of four, five or six consonants are not unusual—for instance, /brtʼqʼɛli/ (flat), /mt͡sʼvrtnɛli/ (trainer) and /prt͡skvna/ (peeling)—and if grammatical affixes are used, it allows an eight-consonant cluster: /ɡvbrdɣvnis/ (he's plucking us), /gvprt͡skvni/ (you peel us). Consonants cannot appear as syllable nuclei in Georgian, so this syllable is analysed as CCCCCCCCVC. Many Slavic languages may manifest almost as formidable numbers of consecutive consonants, such as in the Czech tongue twister Strč prst skrz krk (pronounced [str̩tʃ pr̩st skr̩s kr̩k] ^ⓘ), meaning 'stick a finger through the neck', the Slovak words štvrť /ʃtvr̩c/ ("quarter"), and žblnknutie /ʒbl̩ŋknucɪɛ̯/ ("clunk"; "flop"), and the Slovene word skrbstvo /skrbstʋo/ ("welfare"). However, the liquid consonants /r/ and /l/ can form syllable nuclei in West and South Slavic languages and behave phonologically as vowels in this case.

An example of a true initial cluster is the Polish word wszczniesz (/fʂt͡ʂɲɛʂ/ ("you will initiate"). In the Serbo-Croatian word opskrbljivanje /ɔpskr̩bʎiʋaɲɛ/ ("victualling") the ⟨lj⟩ and ⟨nj⟩ are digraphs representing single consonants: [ʎ] and [ɲ], respectively. In Dutch, clusters of six or even seven consonants are possible (e.g. angstschreeuw ("a scream of fear"), slechtstschrijvend ("writing the worst") and zachtstschrijdend ("treading the most softly")).