Ef (Ф ф; italics: Ф ф) is a letter of the Cyrillic script. It commonly represents the voiceless labiodental fricative /f/, like the pronunciation of ⟨f⟩ in "fill". The Cyrillic letter Ef is romanized as ⟨f⟩.

The Cyrillic letter Ef was derived from the Greek letter Phi (Φ φ). It replaced Fita (Ѳ) in the Russian alphabet in 1918.

The name of Ef in the Early Cyrillic alphabet was фрьтъ (frtŭ).

In the Cyrillic numeral system, Ef had a value of 500.

The Slavic languages practically do not have native words containing /f/; this sound, which did not exist in Proto-Indo-European, arose in Greek and Latin from PIE *bʰ (which yielded Slavic /b/) and in the Germanic languages from PIE *p (which remained unchanged in Slavic). The letter ф is, therefore, almost exclusively found in words of foreign origin, especially Greek (from both th and ph), Latin, French, German, English, and Turkic. Few native Slavic words with this letter (in different languages) are examples of onomatopoeia (like Russian verbs фукать, фыркать etc.) or reflect sporadic pronunciation shifts, for example пв /pv/ in Serbian уфати (from Church Slavonic уповати), and хв /xv/, or х /x/ in the Russian toponym Фили (from хилый). There are some examples of native Slavic words (and variations) which include and use the Ф letter natively, like фати or сфати in Macedonian (which represent the Macedonian sound change from the Old Slavic "hv" to "f").