Note: This page or section contains IPA phonetic symbols in Unicode. See International Phonetic Alphabet for a pronunciation key.
The Portuguese alphabet consists of the following 23 letters:
A, B, C, D, E, F, G, H, I, J, L, M, N, O, P, Q, R, S, T, U, V, X, Z
Although not used in vernacular terms, the letters K, W, and Y may appear in personal names, and in words derived from them. Apart from these, Portuguese makes use of the following characters with diacritics:
Á, Â, Ã, À, Ç, É, Ê, Í, Ó, Ô, Õ, Ú, Ü
The latter are not regarded as letters, and they do not have entries of their own in Portuguese dictionaries. When two words differ only in the presence or absence of a diacritic, the one without it is collated first. The trema on Ü is currently only used in Brazilian Portuguese.

Letter names and pronunciations
These are pairs of letters which represent a single sound, different from the sum of their components. They are always pronounced the same way.

Grapheme	Pronunciation
ch	/ʃ/
lh	/ʎ/
nh	/ɲ/
rr	/ʁ/
ss	/s/

The digraph ch is pronounced as an English sh. The digraphs lh and nh, of Occitan origin, denote palatal consonants which do not exist in English, but can be approximated by li, ni in words such as million, onion, pronounced quickly. The digraphs rr and ss are only used between vowels. The pronunciation of the digraph rr varies with dialect (see the note on the phoneme /ʁ/, above).

Basic digraphs
Portuguese makes use of six diacritics to expand the Latin alphabet. Letters with diacritics are not included in the alphabet.

Grapheme	Pronunciation
á	/a/
â	/ɐ/
ã	/ɐ̃/
à	/a/
ç	/s/
é	/ɛ/
ê	/e/
í	/i/
ó	/ɔ/
ô	/o/
õ	/õ/
ú	/u/
ü	/u/

As in other languages of western Europe, the letter u is normally silent in the graphemes gu and qu before a front vowel:
There are, however, a few such words in which the vowel u is pronounced. These exceptions are indicated with a trema (güe, güi, qüe, qüi) in the Brazilian spelling, but not in the European orthography. Most of them are learned latinisms, such as freqüência/frequência "frequency", argüição/arguição "questioning", qüinqüelíngüe/quinquelingue "in five languages" (conjectured to be the Portuguese word with the most diacritics).
The pronunciation of accented letters is fairly stable, except that the vowels are nasalized before syllable-final m or n. See the section on finals, below.

The cedilla indicates that ç is pronounced /s/, due to a historical palatalization.
The acute accent and the circumflex accent mark stressed vowels, and also indicate their relative height in some cases: â, ê, and ô are higher vowels than á, é, ó, respectively.
The tilde (ã, õ) marks nasal vowels. In the last syllable of a word, it also indicates stress, provided there is no acute or circumflex in a previous syllable of the same word. Out of the last syllable, the tilde does not mark primary stress, though it usually coincides with a secondary stress.
The grave accent (à) indicates that a vowel is low without implying primary stress, but may coincide with a secondary stress.
gu is pronounced /g/ before e or i, and /gu/ elsewhere;
qu is pronounced /k/ before e or i, and /ku/ elsewhere. Diacritics

Oral
Grapheme	Pronunciation	Grapheme	Pronunciation
ai, ái	/ai ~ ɐi/	éu	/ɛu/
oi	/oi/	ou	/ou ~ o/
ói	/ɔi/	-
ui	/ui/	iu	/iu/
Nasal
Grapheme	Pronunciation	Grapheme	Pronunciation
ãe, ãi	/ɐ̃ĩ/	ão	/ɐ̃ũ/
õe	/õĩ/	-

The pronunciation of each diphthong is fairly predictable, but one must know how to distinguish true diphthongs from adjacent vowels in hiatus, which belong to separate syllables. For example, in the word saio /ˈsai̯u/, the i forms a diphthong with the previous vowel, but in saiu, /saˈiu̯/ or /sɐˈiu̯/, it forms a diphthong with the next vowel. As in Spanish, a hiatus may be indicated with an acute accent, distinguishing homographs such as saia /ˈsai̯ɐ/ and saía, /saˈiɐ/ or /sɐˈiɐ/.

When followed by a vowel before the stressed syllable, in central Portugal.
In central Portugal. Diphthongs
Some letter pairs have special pronunciations at the end of syllables or words. When a syllable ends with m or n, this consonant is not fully pronounced, but merely indicates the nasalization of the vowel which precedes it. At the end of words, this sometimes produces a nasal diphthong.

Monophthongs		Diphthongs
Grapheme	Pronunciation	Grapheme	Pronunciation
-an, -am, -ân, -âm	/ũ/

The grapheme -en- is also pronounced as a nasal diphthong in a few compound words, such as bendito (bem + dito), homenzinho (homem + zinho), and Benfica.

at the end of a syllable
at the end of a word
at the end of a syllable or word
before final s, for example in the word parabéns Finals
The graphemes sç and xs are pronounced as one sound /s/ in BP, but as two sounds /ʃs/ in standard EP. In BP, the letter pairs sc and xc are also pronounced /s/ before e or i. In standard EP, they are pronounced /ʃs/.
In the European orthography, the letters c and p are sometimes silent before c, ç, or t, especially in latinisms or hellenisms.

Other silent letters
The letter y was never used consistently in medieval Portuguese. During the Renaissance, some authors reintroduced it in words of Latin or Greek origin, for etymology, or as a semivowel in falling diphthongs, like in Spanish. The Portuguese spelling reform of 1911, and the later spelling convention signed between Portugal and Brazil in 1931, however, abolished etymological spellings and decreed that semivowel y should be written i, since it is an allophone of the vowel /i/.
The letters k and w were always uncommon in Portuguese spelling, although they appeared occasionally in some proper nouns. Nevertheless, the use of these three letters is allowed in hybrid words derived from foreign names, such as keynesiano and newtoniano, listed even in the most authoritative Portuguese dictionaries. They are sorted as in English.

Personal names
The Vietnamese orthography is based on the orthography of Portuguese, through the work of 16th century Catholic missionaries.

Trivia
There are two QWERTY-based keyboard layouts used for Portuguese.
Additionally, there are two variants of the Dvorak Simplified Keyboard designed for Brazilian Portuguese.
Typewriters in Portuguese-speaking countries generally have a separate extra key for Ç, and a dead key for each diacritic except the cedilla; so that Á is obtained by typing first the acute accent, then the letter A. The same thing happens with computer keyboards, except when using an "English - International" keyboard layout, where to type Ç one should first type the acute accent and then the letter C.
The JLG Extended Keyboard Layout for US is a freeware that allows writing in Portuguese from a US QWERTY keyboard layout. More information at the QWERTY and accents article.

Notes

Estrela, Edite A questão ortográfica — Reforma e acordos da língua portuguesa (1993) Editorial Notícias
Full text of the Pequeno Vocabulário Ortográfico da Língua Portuguesa (Abridged Ortographic Vocabulary of the Portuguese Language) published by the Brazilian Academy of Letters in 1943.
Text of the decree of the Brazilian government, in 1971, amending the ortography adopted in 1943 (no updated version of the PVOLP was published).
IILP — Orthographic Agreement of 1990 (PDF - in Portuguese)