U (У у; italics: У у or У у; italics: У у) is a letter of the Cyrillic script. It commonly represents the close back rounded vowel /u/, somewhat like the pronunciation of ⟨oo⟩ in "boot" or "rule". The forms of the Cyrillic letter U are similar to the lowercase of the Latin letter Y (Y y; Y y), with the lowercase Cyrillic letter U's form being identical to that of small Latin letter Y.

Historically, Cyrillic U evolved as a specifically East Slavic short form of the digraph ⟨оу⟩ used in ancient Slavic texts to represent /u/. The digraph was itself a direct loan from the Greek alphabet, where the combination ⟨ου⟩ (omicron-upsilon) was also used to represent /u/. Later, the o was removed, leaving the modern upsilon-only form.

Consequently, the form of the letter is derived from Greek upsilon ⟨Υ υ⟩, which was parallelly also taken over into the Cyrillic alphabet in another form, as Izhitsa ⟨Ѵ⟩. (The letter Izhitsa was removed from the Russian alphabet in the orthography reform of 1917/19.)

It is normally romanised as "u", but in Kazakh, it is romanised as "w".

In the Cyrillic numeral system, the Cyrillic letter U had a value of 400.

In Tuvan the Cyrillic letter can be written as a double vowel.

In certain languages, U is used to mark labialization.