Pataḥ (Hebrew: פַּתָּח patákh, IPA: [paˈtaħ], Biblical Hebrew: pattā́ḥ) is a Hebrew niqqud vowel sign represented by a horizontal line ⟨ אַ‎ ⟩ underneath a letter. In modern Hebrew, it indicates the phoneme /a/ which is close to the "[a]" sound in the English word far and is transliterated as an a.

In Modern Hebrew, a pataḥ makes the same sound as a qamatz, as does the ḥaṭaf pataḥ (Hebrew: חֲטַף פַּתַח IPA: [ħaˈtˤaf paˈtaħ], "reduced pataḥ"). The reduced (or ḥaṭaf) niqqud exist for pataḥ, qamatz, and segol which contain a shva next to it.

In Yiddish orthography, a pataḥ (called pasekh in Yiddish) has two uses. The combination of pasekh with the letter aleph, אַ, is used to represent the vowel [a]; the combination of pasekh with a digraph consisting of two yods, ײַ, is used to represent the diphthong [aj].

The following table contains the pronunciation and transliteration of the different pataḥs in reconstructed historical forms and dialects using the International Phonetic Alphabet.

The letters Bet ⟨ב‎⟩ and Het ⟨ח‎⟩ used in this table are only for demonstration, any letter can be used.

A pataḥ on a letter ח‎, ע‎, or הּ‎ (that is, ה‎ with a dot (mappiq) in it) at the end of a word is sounded before the letter, and not after. Thus, נֹחַ (Noah; properly transliterated as Noaḥ) is pronounced /no.aχ/ in Modern Hebrew and /no.aħ/ or /no.ʔaħ/ in Biblical Hebrew. This only occurs at the ends of words, only with pataḥ and only with these three letters. This is sometimes called a pataḥ gnuva, or "stolen" pataḥ (more formally, "furtive pataḥ"), since the sound "steals" an imaginary epenthetic consonant to make the extra syllable.

By adding two vertical dots (shva) the vowel is made very short. However, these vowels lengths are not manifested in Modern Hebrew.

• Niqqud
• Qamatz
• Fathah, the related diacritic in Arabic