Iota (upper case Ι, lower case ι) is the ninth letter of the Greek alphabet. In the system of Greek numerals it has a value of 10.

Iota is pronounced like a 'continental' i or English ee. In ancient Greek it occurred in both long and short versions, but this distinction has been lost in Modern Greek.

Upsilon participated as the second element in falling diphthongs, with both long and short vowels as the first element. Where the first element was long, the iota was lost in pronunciation at an early date, and was written in polytonic orthography as iota subscript in other words as a very small ι under the main vowel, for instance

The word iota is also used in English to express a very small amount, because iota is the smallest letter in the Greek alphabet.

The word is also used in a common English phrase, 'not one iota of difference', to signify a meaningless distinction. The phrase derives from a theological debate which arose around the time of the Nicene Creed, regarding the nature of the Holy Trinity. The argument centered on which of two alternative Greek words, differing only in a single 'iota' letter, should be used in describing Jesus's relationship to the Holy Trinity.

The Iota symbol is used to sort items in the APL programming language.

Letters that arose from Iota include the Roman I.