Finnish orthography is based on the Latin script, and uses an alphabet derived from the Swedish alphabet, officially comprising twenty-nine letters but also including two additional letters found in some loanwords. The Finnish orthography strives to represent all morphemes phonologically and, roughly speaking, the sound value of each letter tends to correspond with its value in the International Phonetic Alphabet (IPA) – although some discrepancies do exist.

The following table describes how each letter in the Finnish alphabet (Finnish: suomen aakkoset) is spelled and pronounced separately. If the name of a consonant begins with a vowel (usually ⟨ä⟩ [æ]), it can be pronounced and spelled either as a monosyllabic or bisyllabic word. In practice, the names of the letters are rarely spelled, as people usually just type the (uppercase or lowercase) glyph when they want to refer to a particular letter.

The pronunciation instructions enclosed in slashes are broad transcriptions based on the IPA system. In notes, more narrow transcriptions are enclosed in square brackets.

The letters ⟨š⟩ and ⟨ž⟩ are variants of ⟨s⟩ and ⟨z⟩, but they are often overlooked, as they are only used in some relatively new loanwords and foreign names, and may be replaced with ⟨sh⟩ and ⟨zh⟩, respectively, if it is technically impossible to reproduce ⟨š⟩ and ⟨ž⟩. The Finnish keyboard layout on Microsoft Windows does not include ⟨š⟩ or ⟨ž⟩; thus, in practice, only highly formal sources such as official texts, encyclopedias or Helsingin Sanomat use them.

The main peculiarities in the Finnish alphabet are the two extra vowels ⟨ä⟩ and ⟨ö⟩ (and Swedish ⟨å⟩, which is not actually needed in Finnish). In Finnish, these extra letters are collectively called ääkköset when they need to be distinguished from the ISO basic Latin alphabet; the word is a somewhat playful modification of aakkoset, which is Finnish for "alphabet". Another informal term is skandit or skandimerkit, short for skandinaaviset merkit "Scandinavian characters" (however, the Danish and Norwegian ⟨æ⟩ and ⟨ø⟩ are usually not taken into account).

In Finnish, ⟨ä⟩, ⟨ö⟩ and ⟨y⟩ are the "front vowel" counterparts to the "back vowels" ⟨a⟩, ⟨o⟩ and ⟨u⟩; grammatical endings and suffixes using these letters, use either the front or back form depending on the vowel harmony of the word. The glyphs for ⟨ä⟩ and ⟨ö⟩ are derived from the similar-looking German umlauted letters, but as with ⟨y⟩ versus ⟨u⟩, they are considered letters in their own right and thus alphabetized separately (after ⟨z⟩).

The Germanic umlaut or convention of considering digraph ⟨ae⟩ equivalent to ⟨ä⟩, and ⟨oe⟩ equivalent to ⟨ö⟩ is inapplicable in Finnish. Moreover, in Finnish, both ⟨ae⟩ and ⟨oe⟩ are vowel sequences, not single letters, and they have independent meanings (e.g. haen "I seek" vs. hän "he, she").

In handwritten text, the actual form of the extra marking may vary from a pair of dots to a pair of short vertical bars, to a single horizontal bar, or to a wavy line resembling a tilde. In practice, almost any diacritic situated above the base glyph (such as, á ā ã) would probably be interpreted as a carelessly written pair of dots (ä). However, in computerized character sets, these alternatives are incorrect. The front-vowel counterpart of ⟨u⟩ using ⟨y⟩ rather than ⟨ü⟩ is carried over from Swedish, and also avoids confusion in cursive script with ⟨ii⟩, which is common in Finnish.