Tamil phonology is characterised by the presence of "true-subapical" retroflex consonants and multiple rhotic consonants. Its script does not distinguish between voiced and unvoiced consonants; phonetically, voice is assigned depending on a consonant's position in a word, voiced intervocalically and after nasals except when geminated. Tamil phonology permits few consonant clusters, which can never be word initial.

The vowels are called உயிரெழுத்து uyireḻuttu ('life letter'). The vowels are classified into short and long (five of each type) and two diphthongs.

The long (netil) vowels are about twice as long as the short (kuṟil) vowels. The diphthongs are usually pronounced about 1.5 times as long as the short vowels, though most grammatical texts place them with the long vowels.

Tamil has two diphthongs: /aɪ̯/ ஐ and /aʊ̯/ ஔ, the latter of which is restricted to a few lexical items. Some like Krishnamurti consider the diphthongs as clusters of /a/ + /j, ʋ/ as they pattern with other VC. The way some words are written also varies e.g. avvai as அவ்வை (avvai), ஔவை (auvai) or அவ்வய் (avvay) (first one most common). Word final /u/ is pronounced as [ɯ~ɨ], it is called a குற்றியலுகரம் (kuṟṟiyalukaram) "short u" (as it has only half a sound unit, compared to 1, 1.5 or 2 of other vowels) in tolkāppiyam and it is unrounded even in literary Tamil; in spoken Tamil it can occur medially as well in some words after the first syllable. Word final [u] occurs in some names, chiefly nicknames like rājēndraṉ as rāju but also some other terms of the form (C)VCu as in karu, puḻu. There is a minimal pair naṭu "center", naṭŭ "to plant". In declensions the full u isnt deleted like the half u like naṭuvillai - naṭillai.

Colloquially, an initial /i(:)/ or /e(:)/ may have a [ʲ] onglide; likewise, an initial /o(:)/ or /u(:)/ may have a [ʷ] onglide, e.g. [ʲeɾi] and [ʷoɾɯ]. In Karnatakan dialects short versions of them may further become [a, ʋa], eg. eṉṟu, utai /ɐnnʉ, ʋɐd̪iki/. This is very light or doesn't happen in Sri Lankan dialects.

Indian Colloquial Tamil also has nasalized vowels formed from word final vowel + nasal cluster (except for /Vɳ/ where an epenthetic u is added after it). Long vowel + nasal just nasalizes the vowel, short vowel + nasal may also change the quality, for example, /an/ gets fronted to [ɛ̃] அவன் /aʋan/ becomes [aʋɛ̃] ([aʋæ̃] for some speakers), /am/ gets rounded to [õ] மரம் /maɾam/ becomes [maɾõ], நீங்களும் /niːŋkaɭum/ becomes [n̪iːŋɡaɭũ], வந்தான் /ʋan̪t̪a:n/ becomes [ʋan̪d̪ã:], the remaining vowels only get nasalized. There is word final nasals in loanwords like ṭaim, ōm, sāmān. Karnataka's dialects have [ʊ̃] for /an/ and -m is just deleted, eg. maram [mɐrʊ].

In spoken Tamil sometimes an epenthetic vowel u is added to words ending in consonants, e.g. nil > nillu, āḷ > āḷu, nāḷ > nāḷu (nā in some dialects), vayal > vayalu etc. If another word is joined at the end, it is deleted.

Colloquially, the high short vowels /i/, /u/ are lowered to [e] and [o] when next to a short consonant and /a, aɪ/. For example, இடம் /iʈam/ becomes [eɖam]; and உடம்பு /uʈampu/ becomes [oɖambɯ]. This is an instance of raising umlaut. It doesn't happen in pronouns and some other words e.g. இவன் ivaṉ and எவன் evaṉ are different words. /aɪ/ also monophthongises to an /e, a/ but it causes the lowering of /i, u/ before it, e.g. ilai > ele/ela. Additionally, the front long vowels /eː/ and /iː/ are subject to retraction when present in the first syllable of a bisyllabic word and followed by a retroflex consonant. As such, /ʋiːʈu/ "house" becomes [ʋɨːɖɪ̈], but its inflected form /ʋiːʈʈukku/ remains [ʋiːʈ(ː)uk(ː)ɪ̈]. Likewise, /t̪eːʈu/ "search!" becomes [t̪əːɖɪ̈], but /t̪eːʈinaːn/ "(he) searched" remains [t̪eːɖinãː]. The presence and degree of retraction for each vowel may be different; it varies between dialects and even individual speakers. Almost all words end with vowels in spoken Tamil.