The Niihau dialect (, ) is a variety of the Hawaiian language spoken on the island of Niûihau, more specifically in its only settlement Puûuwai, and on the island of Kauaûi around Kekaha where descendants of families from Niihau now live. Today, the Niihau variety of Hawaiian is taught in Ke Kula Niihau O Kekaha.
Today, families with ancestry in Niihau who now on western Kauaûi speak a similar variety to that spoken on Niihau, but some speakers refer to the speakers of the dialect outside of Niûihau as speakers of Olelo Kauaûi.
Unlike the Hawaiian taught in schools, the Niûihau dialect maintains the variation between and , in addition to and . Some other pockets of speakers on Molokai and Maui have also been found to maintain the variant. While in the 1950s the Niûihau dialect had free variation between and , recent observations suggest that and are currently found in largely complementary distribution in the modern Niûihau dialect. The allophone appears when before other syllables containing the allophone: thus Niûihau has 'one', 'we (inclusive)', 'year', where standard Hawaiian has , , and .
This pattern of dissimilation is also extended to some loanwords. For example, the English word 'cook' is reflected in Niûihau Hawaiian as , even though the word 'cook' does not have a in English.
The allophone, represented in standard Hawaiian and the Hawaiian alphabet, is prestigious and associated with reading styles. The Bible in particular is always read with . The dissimilation pattern in colloquial Niûihau may be due to an effort to preserve the Niûihau dialect's distinctiveness from standard Hawaiian.
Like the Hawaiian taught in universities, ÃȈ Âlelo Niûihau has five short and five long vowels, plus diphthongs.
Niûihau retains the five pure vowels characteristic of Hawaiian with few changes. The short vowels are , and the long vowels, if they are considered separate phonemes rather than simply sequences of like vowels, are . When stressed, short and have been described as becoming and , while when unstressed they are and . Parker Jones, however, did not find a reduction of /a/ to in the phonetic analysis of a young speaker from Hilo, Hawaiûi; so there is at least some variation in how /a/ is realised. also tends to become next to , , and another , as in Pele . Some grammatical particles vary between short and long vowels. These include a and o "of", ma "at", na and no "for". Between a back vowel or and a following non-back vowel (), there is an epenthetic , which is generally not written. Between a front vowel or and a following non-front vowel (), there is an epenthetic (a y sound), which is never written.
The short-vowel diphthongs are . In all except perhaps , these are falling diphthongs. However, they are not as tightly bound as the diphthongs of English, and may be considered vowel sequences. (The second vowel in such sequences may receive the stress, but in such cases it is not counted as a diphthong.) In fast speech, tends to and tends to , conflating these diphthongs with and .
There are only a limited number of vowels which may follow long vowels, and some authors treat these sequences as diphthongs as well: .
Research done by Newman (1951) suggests Niûihau dialect is among the fastest spoken Hawaiian dialects. He reported a Niûihau woman having a reading speed of 170 words per minute whereas a man from Kalapana read at a slower 120.
The fast pace of the Niûihau dialect causes a number of phonemic reductions. Newman lists three examples of this phenomenon:
Niûihau dialect does not use an ûokina to represent glottal stops nor a kahakà  (macron) to indicate long vowels. The Hawaiian word /ÃÂoÃÂlelo/ ("language") is spelt olelo in Niûihau and ÃȈ Âlelo in Standard Hawaiian.