The soft sign (Ь ь; italics: Ь ь or Ь ь; italics: Ь ь) is a letter in the Cyrillic script that is used in various Slavic languages. In Old Church Slavonic, it represented a short or reduced front vowel. However, over time, the specific vowel sound it denoted was largely eliminated and merged with other vowel sounds.

In most contemporary Slavic Cyrillic writing systems, such as those used in East Slavic languages (Russian, Ukrainian, Belarusian) and Church Slavic, the soft sign does not represent a distinct sound on its own. Instead, it serves as an indicator of palatalization of the preceding consonant. In the Bulgarian language, it is only used to mark the palatalization of the preceding consonant when in front of the letter o, causing the combination ьо (/ʲo/). An example of this is the word гьол (/gʲol/).

Palatalization is a linguistic process in which the middle of the tongue moves closer to the hard palate while pronouncing a consonant. It affects the pronunciation of the preceding consonant by giving it a palatal quality or causing it to become a palatal consonant. The soft sign acts as a visual marker to show that the consonant before it is palatalized.

For example, in Russian, the soft sign is often used after consonants to indicate palatalization. It affects the pronunciation of the preceding consonant and can change the sound of words. The specific effect varies depending on the consonant it follows. In other Slavic languages where the soft sign is used, a similar palatalization function is observed.

It may also be used as a superscript (ꚝ) in Lithuanian dialectology.

The soft sign is normally written after a consonant and indicates its softening (palatalization) (for example Ukrainian батько 'father'). Less commonly, the soft sign just has a grammatically determined usage with no phonetic meaning (like Russian: туш 'fanfare' and тушь 'India ink', both pronounced /tuʂ/ but different in grammatical gender and declension). In East Slavic languages and some other Slavic languages (such as Bulgarian), there are some consonants that do not have phonetically different palatalized forms but corresponding letters still admit the affixing soft sign.

The Serbian Cyrillic alphabet has had no soft sign as a distinct letter since the mid-19th century: palatalization is represented by special consonant letters instead of the sign (some of these letters, such as ⟨Љ⟩ or ⟨Њ⟩, were designed as ligatures with the grapheme of the soft sign). The modern Macedonian alphabet, based on the Serbian Cyrillic variant, has had no soft sign since its creation, in 1944.

Between a consonant and a vowel, the soft sign bears also a function of "iotation sign": in Russian, vowels after the soft sign are iotated (compare Russian льют /lʲjut/ '(they) pour/cast' and лют /lʲut/ '(he is) fierce'). The feature, quite consistent with Russian orthography, promulgated a confusion between palatalization and iotation, especially because ⟨ь⟩ usually precedes so-called soft vowels. Combinations ⟨ья⟩ (ya), ⟨ье⟩ (ye), ⟨ьё⟩ (yo) and ⟨ью⟩ (yu) give iotated vowels, like corresponding vowel letters in isolation (and word-initially), and unlike its use immediately after a consonant letter in which palatalization can occur but not iotation. In those cases, ⟨ь⟩ may be considered as a sign indicating that a vowel after it is pronounced separately from the previous consonant, but that is the case neither for ⟨ьи⟩ (yi) nor for ⟨ьо⟩ (yo), because these vowels are not iotated in isolation. The latter case, though, is rarely used in Russian (only in loanwords such as ⟨бульон⟩) and can be seen as a replacement of phonetically identical ⟨ьё⟩, which gets rid of an "inconvenient" letter ⟨ё⟩. In Ukrainian and Bulgarian, the spelling ⟨ьо⟩ indicates palatalization, not iotation.