Nouns are classified into two genders ("masculine" and "feminine") and are inflected for number (singular or plural). Adjectives and determiners (articles, demonstratives, possessives, and quantifiers) must be inflected to agree with the noun in gender and number.

Portuguese does not inflect nouns to indicate their grammatical function or case, relying instead on the use of prepositions (simple and phrasal), on pleonastic objects, or on the context or word order.

Gender and number

Most adjectives and demonstratives, and all articles must be inflected according to the gender and number of the noun they reference:

   esta linda casa branca ("this lovely white house")
   este lindo carro branco ("this lovely white car")
   estas lindas aves brancas ("these lovely white birds")
   estes lindos gatos brancos ("these lovely white cats")

Plural formation

Portuguese nouns form their plurals by adding -s if the singular ends in a vowel, and -es if the singular ends in n, r or z. If the singular ends in s, then if the last syllable is stressed, the plural adds -es, and otherwise the plural is the same as the singular. Words ending in m change that m into ns, and words ending in l change that l into -is (e.g. animal > animais). Words ending in ão vary in how they form their plurals: some replace the ão with ães, others with ões, and others just add an -s like the other nouns ending in a vowel.

Gender determination

Grammatical gender of inanimate entities is often different from that used in sister languages: thus, for example, Portuguese árvore ("tree") and flor ("flower") are feminine, while Spanish árbol and Italian fiore are masculine; Portuguese mar ("sea") and mapa ("map") are masculine, while French mer and mappe are feminine.

In many cases, the gender and number of a noun can be deduced from its ending: the basic pattern is "-o" / "-os" for masculine singular and plural, "-a" / "-as" for feminine. So, casa ("house"), mala ("suitcase"), pedra ("stone"), and inteligência ("intelligence") are feminine, while carro ("car"), saco ("bag"), tijolo ("brick"), and aborrecimento ("annoyance") are masculine. However, the complete rules are quite complex: for instance, nouns ending in -ção are usually feminine, except for augmentatives like bração ("big arm"). And there are many irregular exceptions. For words ending in other letters, there are few rules: flor ("flower"), gente ("folk"), nau ("ship"), maré ("tide") are feminine, while amor ("love"), pente ("comb"), pau ("stick"), café ("coffee") are masculine.

On the other hand, the gender of some nouns, as well as of first- and second-person pronouns, is determined semantically by the sex or gender of the referent: aquela estudante é nova, mas aquele estudante é velho ("this (female) student is new, but that (male) student is old"); or eu sou brasileiro ("I am Brazilian", said by a man) and eu sou brasileira (the same, said by a woman).

Also, many animate masculine nouns have specific feminine derivative forms to indicate female sex or social gender: lobo ("wolf" or "male wolf", masculine gender) → loba ("she-wolf", feminine), conde ("count", m.) → condessa ("countess", f.), doutor ("doctor" or "male doctor", m.) → doutora ("female doctor", f.), ator ("actor", m.) → atriz ("actress", f.), etc.