Diacritics are used with the Thai alphabet to indicate modifications of the values of the letters.

Thai is a tonal language, and the script gives full information on the tones. Tones are realised in the vowels, but indicated in the script by a combination of the class of the initial consonant (high, mid or low), vowel length (long or short), closing consonant (unvoiced-plosive or voiced-sonorant) and, if present, one of four tone marks. The names and signs of the tone marks are derived from the numbers one, two, three and four in an Indic language.

Two consonant characters (not diacritics) are used to modify the tone:

ห นำ ho nam, leading ho. A silent, high-class ห "leads" low-class nasal consonants (ง, ญ, น and ม) and non-plosives (ว, ย, ร and ล), which have no corresponding high-class phonetic match, into the tone properties of a high-class consonant. In polysyllabic words, an initial mid- or high-class consonant with an implicit vowel similarly "leads" these same low-class consonants into the higher class tone rules, with the tone marker borne by the low-class consonant.

อ นำ o nam, leading o. In four words only, a silent, mid-class อ "leads" low-class ย into mid-class tone rules: อย่า (ya, don't) อยาก (yak, desire) อย่าง (yang, yet) อยู่ (yu, stay). Note all four have long-vowel, low-tone siang ek, but อยาก, a dead syllable, needs no tone marker, but the three live syllables all take mai ek.

Exceptions where words are spelled with one tone but pronounced with another often occur in informal conversation (notably the pronouns ฉัน chan and เขา khao, which are both pronounced with a high tone rather than the rising tone indicated by the script). Generally, when such words are recited or read in public, they are pronounced as spelled.

Other diacritics are used to indicate short vowels and silent consonants:

–็ Mai taikhu (to indicate short vowels) means "stick that climbs and squats". It is a miniature Thai numeral 8 ๘. Mai taikhu is often used with sara e (เ) and sara ae (แ) in closed syllables.

–์ Thanthakhat (to indicate silent consonants) means "killing as punishment"; also called karan, meaning "canceled".