A phonetic complement is a phonetic symbol used to disambiguate word characters (logograms) that have multiple readings, in mixed logographic-phonetic scripts such as Egyptian hieroglyphs, Akkadian cuneiform, Linear B, Japanese, and Mayan. Often they disambiguate an ideogram by spelling out the first or last syllable of the word; occasionally (as in Linear B) they may instead abbreviate an adjective that modifies the logogram.

Written English has few logograms, primarily numerals, and therefore few phonetic complements. An example is the nd of 2nd 'second', which avoids ambiguity with 2 standing for the word 'two'. In addition to numerals, other examples include Xmas, Xianity, and Xing for Christmas, Christianity, and Crossing – note the separate readings Christ and Cross.

In Sumerian, the single word kur (𒆳) had two main meanings: 'hill' and 'country'. Akkadian, however, had separate words for these two meanings: šadú 'hill' and mātu 'country'. When Sumerian cuneiform was adapted (known as orthographic borrowing) for writing Akkadian, this was ambiguous because both words were written with the same character (𒆳, conventionally transcribed KUR, after its Sumerian pronunciation). To alert the reader as to which Akkadian word was intended, the phonetic complement -ú was written after KUR if 'hill' was intended, so that the characters KUR-ú were pronounced šadú, whereas KUR without a phonetic complement was understood to mean mātu 'country'.

Phonetic complements also indicated the Akkadian nominative and genitive cases. Similarly, Hittite cuneiform occasionally uses phonetic complements to attach Hittite case endings to Sumerograms and Akkadograms.

Phonetic complements should not be confused with determinatives (which were also used to disambiguate) since determinatives were used specifically to indicate the category of the word they preceded or followed. For example, the sign DINGIR (𒀭) often precedes names of gods, as LUGAL (𒈗) does for kings. It is believed that determinatives were not pronounced.

As in Akkadian, Japanese borrowed a logographic script, Chinese, designed for a very different language. The Chinese phonetic components built into these kanji (Japanese: 漢字) do not work when they are pronounced in Japanese, and there is not a one-to-one relationship between them and the Japanese words they represent.

For example, the kanji 生, pronounced shō or sei in borrowed Chinese vocabulary, stands for several native Japanese words as well. When these words have inflectional endings (verbs/adjectives and adverbs), the end of the stem is written phonetically:

as well as the hybrid Chinese-Japanese word