Japanese phonology is the system of sounds used in the pronunciation of the Japanese language. Unless otherwise noted, this article describes the standard variety of Japanese based on the Tokyo dialect.

There is no overall consensus on the number of contrastive individual sounds (phonemes). Common approaches recognize at least 12 distinct consonants (as many as 21 in some analyses) and 5 distinct vowels, /a, e, i, o, u/. Phonetic length is contrastive for both vowels and consonants, and the total length of Japanese words can be measured in a unit of timing called the mora (from Latin mora "delay"). Only limited types of consonant clusters are permitted. There is a pitch accent system where the position or absence of a pitch drop may determine the meaning of a word: /haꜜsiɡa/ (箸が, 'chopsticks'), /hasiꜜɡa/ (橋が, 'bridge'), /hasiɡa/ (端が, 'edge').

Japanese phonology has been affected by the presence of several layers of vocabulary in the language. In addition to native Japanese vocabulary, Japanese has a large amount of Chinese-based vocabulary (used especially to form technical and learned words, playing a similar role to Latin-based vocabulary in English) and loanwords from other languages. Different layers of vocabulary allow different possible sound sequences (phonotactics).

Many generalizations about the sound system of Japanese have exceptions when recent loanwords are taken into account. For example, the consonant [p] generally does not occur at the start of native (Yamato) or Chinese-derived (Sino-Japanese) words, but it occurs freely in this position in mimetic and foreign words. Because of exceptions like this, discussions of Japanese phonology often refer to layers, or "strata," of vocabulary. The following four strata may be distinguished:

Called wago (和語) or yamato kotoba (大和言葉) in Japanese, this category consists of inherited native vocabulary. Morphemes in this category show a number of restrictions on structure that may be violated by vocabulary in other layers.

Japanese possesses a variety of mimetic words that make use of sound symbolism to serve an expressive function. Like Yamato vocabulary, these words are also of native origin, and can be considered to belong to the same overarching group. However, words of this type show some phonological peculiarities that cause some theorists to regard them as a separate layer of Japanese vocabulary.

Called kango (漢語) in Japanese, words in this stratum originate from several waves of large-scale borrowing from Chinese that occurred from the 6th-14th centuries AD. They comprise 60% of dictionary entries and 20% of ordinary spoken Japanese, ranging from formal vocabulary to everyday words. Most Sino-Japanese words are composed of more than one Sino-Japanese morpheme. Sino-Japanese morphemes have a limited phonological shape: each has a length of at most two moras, which Ito & Mester (2015a) argue reflects a restriction in size to a single prosodic foot. These morphemes represent the Japanese phonetic adaptation of Middle Chinese monosyllabic morphemes, each generally represented in writing by a single Chinese character, taken into Japanese as kanji (漢字). Japanese writers also repurposed kanji to represent native vocabulary; as a result, there is a distinction between Sino-Japanese readings of kanji, called On'yomi, and native readings, called Kun'yomi.

The moraic nasal /N/ is relatively common in Sino-Japanese, and contact with Middle Chinese is often described as being responsible for the presence of /N/ in Japanese (starting from approximately 800 AD in Early Middle Japanese), although /N/ also came to exist in native Japanese words as a result of sound changes.