Breton is a Brittonic Celtic language in the Indo-European family, and its grammar has many traits in common with these languages. Like most Indo-European languages it has grammatical gender, grammatical number, articles and inflections and, like the other Celtic languages, Breton has mutations. In addition to the singular–plural system, it also has a singulative–collective system, similar to Welsh. Unlike the other Brittonic languages, Breton has both a definite and indefinite article, whereas Welsh and Cornish lack an indefinite article and unlike the other extant Celtic languages, Breton has been influenced by French.

Like in most other Indo-European languages, Breton nouns belong to distinct grammatical genders/noun classes: masculine (gourel) and feminine (gwregel). The neuter (nepreizh), which existed in Breton's ancestor, Brittonic, survives in a few words, such as tra (thing), which takes and causes the mutations of a feminine noun but in all other grammatical respects behaves as if it were masculine.

The gender of a noun is hard to predict, and for some words can even vary from dialect to dialect. However, certain semantic groups of word tend to belong to a particular gender. For example, names of countries and cities often are feminine, whereas most divisions of time are masculine. Some suffixes denote the same gender:

Nouns may exist in as many as four numbers: collective / singulative (see below) or singular / plural. Most plural forms are formed with the addition of a suffix, often -ed for animate nouns and -(i)où for inanimates, for example, Breton "Breton" to Bretoned "Bretons", levr "book" to levroù, although some nouns referring to people take -où, such as test "witness" becoming testoù. Other suffixes also occur, for example, Saoz "Englishman" to Saozaon, ti "house" to tiez. A few nouns form their plural via vowel alternation, such as kastell "castle" to kestell, maen "stone" to mein, the combination of a suffix and vowel alternation, such as bran "crow" to brini, gad "hare" to geden while others are irregular, like den "person" to tud, ki "dog" to either kon or chas.

As well as having a regular plural form, certain parts of the body display relics of a dual system, prefixing daou- to masculine nouns and di(v)- to feminine nouns. An example of this is singular lagad "eye", plural lagadoù "eyes", dual daoulagad "(pair of) eyes". Dual forms themselves can have a plural form, for example, daoulagadoù "(pairs of) eyes".

A distinctive and unusual feature of Brythonic languages is a singulative marker, which in Breton is marked with the feminine suffix -enn. While the collective noun gwez, for instance, means "trees (collectively)", the singulative gwezenn means "(a single) tree". The latter can even be made into a regular plural gwezennoù with the meaning "several trees (individually)".

Breton forms diminutive nouns using the suffix -ig with the plural formed by reduplication of the suffix -où, for example, prad "meadow", pradig "little meadow", pradouigoù "little meadows" (cf. non-diminutive plural pradoù "meadows").

In Breton, the article has both definite and indefinite forms. This is unlike other Celtic languages, which have only definite articles. The definite article is an before dentals, vowels and unpronounced h, al before l and ar elsewhere. Examples of this include an tan "the fire", al logodenn "the mouse", ar gador "the chair". The indefinite article, derived from the number un "one", follows the same pattern of final consonants: un tan "a fire", ul logodenn "a mouse", ur gador "a chair".