Spanish adjectives are similar to those in most other Indo-European languages. They are generally postpositive, and they agree in both gender and number with the noun they modify.

Spanish adjectives can be broadly divided into two groups: those whose lemma (the base form, the form found in dictionaries) ends in -o, and those whose lemma does not. The former generally inflect for both gender and number; the latter generally inflect just for number. Frío ("cold"), for example, inflects for both gender and number. When it is used with a masculine singular noun, the masculine singular form frío (the lemma) is used. When it is used with a feminine singular noun, it becomes fría; -a is generally the feminine singular ending for adjectives that inflect for gender. When it is used with a masculine plural noun, it becomes fríos, and when it is used with a feminine plural noun, it becomes frías; -s is the plural marker for both the masculine and feminine with adjectives that inflect for gender. Thus:

Adjectives whose lemma does not end in -o, however, inflect differently. These adjectives almost always inflect only for number. -s is once again the plural marker, and if the lemma ends in a consonant, the adjective takes -es in the plural. Thus:

This division into two groups is a generalization, however. There are many examples, such as the adjective español itself, of adjectives whose lemmas do not end in -o but nevertheless take -a in the feminine singular as well as -as in the feminine plural and thus have four forms: in the case of español, española, españoles, españolas. There are also adjectives that do not inflect at all (generally words borrowed from other languages, such as the French beige (also Hispanicised to beis)).

Spanish adjectives are very similar to nouns and are often interchangeable with them. Bare adjectives can be used with articles and thus function as nouns where English would require nominalization using the pronoun one(s). For example:

Masculine singular adjectives can also be used with the neuter article lo to signify "the [adjective] thing, the [adjective] part". Thus:

The only inflectionally irregular adjectives in Spanish are those that have irregular comparative forms, and only four do.

Spanish adjectives are generally postpositive, that is, they come after the noun they modify. Thus el libro largo ("the long book"), la casa grande ("the big house"), los hombres altos ("the tall men"), etc. There are, however, a small number of adjectives, including all ordinal numerals as well as words such as otro ("other") and todo ("all"), that must be placed before the noun they modify. There are also a small number that can be placed both before and after the noun and that change meaning according to that positioning, and some adjectives, especially those that form something of a fixed phrase with the noun (e.g. oscura noche ("dark night"), alta montaña ("high mountain")), can be placed before or after the noun with little change in meaning.