The phonology of the Persian language varies between regional dialects and standard varieties. Persian is a pluricentric language and countries that have Persian as an official language have separate standard varieties, namely: Standard Dari (Afghanistan), Standard Iranian Persian (Iran) and Standard Tajik (Tajikistan). The most significant differences between standard varieties of Persian are their vowel systems. Standard varieties of Persian have anywhere from 6 to 8 vowel distinctions, and similar vowels may be pronounced differently between standards. However, there are not many notable differences when comparing consonants, as all standard varieties have a similar number of consonant sounds. Though, colloquial varieties generally have more differences than their standard counterparts. Most dialects feature a contrastive pitch-accent and syllable-final consonant clusters. Linguists tend to focus on Iranian Persian, so this article may contain less adequate information regarding other varieties.

In Iranian Persian there are three short vowels: /æ/, /e/ and /o/, and three long vowels: /ɒː/, /iː/ and /uː/. The three short vowels are only short when in an open syllable (i.e., without a coda) that is non-final (regardless of stress); e.g., صِدا [seˈdɒː] "sound", خُدا [xoˈdɒː] "God". In an unstressed closed syllable, they are around 60 percent as long as a long vowel. Otherwise all vowels are long; e.g., سِفْت‌تَر [seˑfˈtʰtæːɾ] "firmer". When the short vowels are in open syllables, they are also sometimes unstable and may tend to assimilate in quality to the following long vowel (both in informal and formal speech). Thus, دِوِیسْت /deˈviːst/ "two hundred" ranges between [de̝ˈviːstʰ] and [diˈviːstʰ]; شُلُوْغ /ʃoˈluːɢ/ "crowded" ranges between [ʃo̝ˈluːɢ] and [ʃuˈluːɢ]; رَسِیدن /ræsiːˈdæːn/ "to arrive" ranges between [ræ̝siːˈdæːn] and [resiːˈdæːn]; and so on.

In Dari the short vowels are /a/, /ɪ/ and /ʊ/ in Kabul, however /ɪ/ is pronounced as /ɛ/ in other regions such as Herat. In Dari and Tajik /a/ is the most common vowel and at the end of a word may be pronounced as [æ]. Unlike Iranian Persian, Dari has 5 long vowels /ɑː/, /eː/, /iː/, /oː/, and /uː/. The Dari vowel /ɑː/ and the Iranian vowel /ɒː/ are, respectively, the unrounded and rounded versions of the same vowel. ('roundedness' refers to the shape of the lips during pronunciation)

In Iranian Persian word-final /o/ is rare except for تُو [tʰo] "you" and nouns of foreign origin. Word-final /æ/ is very rare in Iranian Persian, with the exception being نَه [næ] "no". The word-final /æ/ in Early New Persian mostly shifted to /e/ in contemporary Iranian Persian, and [e] is also an allophone of /æ/ in word-final position. /e/ is the most common short vowel that is pronounced in final open syllables.

The status of diphthongs in Persian is disputed. Some authors list /ei̯, ou̯, ɒi̯, oi̯, ui̯/, others list only /ei̯/ and /ou̯/, but some do not recognize diphthongs in Persian at all. A major factor that complicates the matter is the change of two classical and pre-classical Persian diphthongs: /ai̯/ > /ei̯/ and /au̯/ > /ou̯/. This shift occurred in Iran but not in some modern varieties (particularly in Afghanistan). Morphological analysis also supports the view that the alleged Persian diphthongs are combinations of the vowels with /j/ and /w/, respectively written as ی and و.

In the modern Perso-Arabic alphabet, the short vowels /a/, /i/ and /u/ are usually left unwritten, as is normally done in the Arabic alphabet. (See Arabic phonology § Vowels.)

Early New Persian inherited from Middle Persian eight vowels: three short i, a, u and five long ī, ē, ā, ō, ū (in IPA: /i a u/ and /iː eː ɑː oː uː/). It is likely that this system passed into the common Persian era from a purely quantitative system into one where the short vowels differed from their long counterparts also in quality: i > [ɪ]; u > [ʊ]; ā > [ɑː]. These quality contrasts have in modern Persian varieties become the main distinction between the two sets of vowels.

The inherited eight-vowel inventory is retained without major upheaval in Dari, which also preserves quantitative distinctions.