Ruby characters or rubi characters (Japanese: ルビ; rōmaji: rubi; Korean: 루비; romaja: rubi) are small, annotative glosses that are usually placed above or to the right of logographic characters of languages in the East Asian cultural sphere, such as Chinese hanzi, Japanese kanji, and Korean hanja, to show the logographs' pronunciation; these were formerly also used for Vietnamese chữ Hán and chữ Nôm, and may still occasionally be seen in that context when reading archaic texts. Typically called just ruby or rubi, such annotations are most commonly used as pronunciation guides for characters that are likely to be unfamiliar to the reader.

Here is an example of Japanese ruby characters (called furigana) for Tokyo ("東京"):

Most furigana are written with the hiragana syllabary, but katakana and romaji are also occasionally used. Alternatively, sometimes foreign words (usually English) are printed with furigana to provide the meaning, and vice versa. Textbooks sometimes render on-readings with katakana and kun-readings with hiragana.

Here is an example of ruby characters for Beijing ("北京") in Zhuyin (a.k.a. Bopomofo), Xiao'erjing, and Pinyin.

In Taiwan, the main syllabary used for Chinese ruby characters is Zhuyin fuhao (also known as Bopomofo); in mainland China pinyin is mainly used. Typically, unlike the example shown above, zhuyin is used with a vertical traditional writing and zhuyin is written on the right side of the characters. In mainland China, horizontal script is used and ruby characters (pinyin) are written above the Chinese characters.

Xiao'erjing is a Perso-Arabic alphabet, adopted by Hui Muslims and at times utilized as ruby characters in various manuscripts. This system does have its shortcomings, mainly that it has no way of indicating tones. With the spread of pinyin, the usage of this system has been in decline in the past decades. Most manuscripts that do mark the characters with Xiao'erjing, do so from right-to-left, which is quite unique, compared to other systems. This is because usually such manuscripts include Arabic texts such as the Quran, and the Chinese writing is the explanation or translation.

Books with phonetic guides (especially pinyin) are popular with children and foreigners learning Chinese.

Here is an example of the Korean ruby characters for Korea ("韓國"):