Upsilon (upper case Υ, lower case υ) is the 20th letter of the Greek alphabet. In the system of Greek numerals it has a value of 400. Four letters of the Latin alphabet arose from it; U, V, W, and Y.

In early Greek it was pronounced like "continental" u or English oo. In Classical Greek, it was pronounced like French u or German ü — a sound that is not found in most dialects of English. In Modern Greek it is pronounced like "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.

As an initial letter in Classical Greek it always carried the rough breathing (equivalent to h) as reflected in the many Greek-derived English words starting with hy.

Upsilon participated as the second element in falling diphthongs, which have subsequently developed in various ways: for instance after alpha or epsilon it is pronounced f or v.

The Roman Emperor Claudius proposed introducing a new letter into the Latin alphabet to approximate the sound of upsilon.

The name of the letter was originally just υ. It changed to "u psilon" (Greek υ ψιλόν, meaning "simple u") to distinguish it from οι, which had come to have the same ü pronunciation.