Latin declension is the set of patterns according to which Latin words are declined—that is, have their endings altered to show grammatical case, number and gender. Nouns, pronouns, and adjectives are declined (verbs are conjugated), and a given pattern is called a declension. There are five declensions, which are numbered and grouped by ending and grammatical gender. Each noun follows one of the five declensions, but some irregular nouns have exceptions.

Adjectives are of two kinds: those like bonus, bona, bonum 'good' use first-declension endings for the feminine, and second-declension for masculine and neuter. Other adjectives such as celer, celeris, celere belong to the third declension. There are no fourth- or fifth-declension adjectives.

Pronouns are also of two kinds, the personal pronouns such as ego 'I' and tū 'you (sg.)', which have their own irregular declension, and the third-person pronouns such as hic 'this' and ille 'that' which can generally be used either as pronouns or adjectivally. These latter decline in a similar way to the first and second noun declensions, but there are differences; for example the genitive singular ends in -īus or -ius instead of -ī or -ae.

The cardinal numbers ūnus 'one', duo 'two', and trēs 'three' also have their own declensions (ūnus has genitive -īus like a pronoun). However, numeral adjectives such as bīnī 'a pair, two each' decline like ordinary adjectives.