Although Portuguese and Spanish are closely related, to the point of having a considerable degree of mutual intelligibility, there are also important differences between them, which can pose difficulties for people acquainted with one of the languages who attempt to learn the other. Both are part of a broader group known as West Iberian Romance, which also contains several minor languages or dialects with fewer speakers, all of which are mutually intelligible among themselves to some degree.

Portuguese and Spanish share a great number of words that are either spelled identically (although they may be pronounced quite differently), almost identically (though they may be pronounced more or less the same), or are similarly predictable. Consider for example the following paragraph, taken from the Gramática esencial del español, by Manuel Seco (Espasa Calpe, 1989), and compare it to the Portuguese rendition below, noting the extensive lexical similarity and the only slight changes in word order:

Pero, a pesar de esta variedad de posibilidades que la voz posee, sería un muy pobre instrumento de comunicación si no contara más que con ella. La capacidad de expresión del hombre no dispondría de más medios que la de los animales. La voz, sola, es para el hombre apenas una materia informe, que para convertirse en un instrumento perfecto de comunicación debe ser sometida a un cierto tratamiento. Esa manipulación que recibe la voz son las "articulaciones".

Porém, apesar desta variedade de possibilidades que a voz possui, seria um instrumento de comunicação muito pobre se não se contasse com mais do que ela. A capacidade de expressão do homem não disporia de mais meios que a dos animais. A voz, sozinha, é para o homem apenas uma matéria informe, que para se converter num instrumento perfeito de comunicação deve ser submetida a um certo tratamento. Essa manipulação que a voz recebe são as "articulações".

There are also some significant differences between Brazilian Portuguese and European Portuguese as there are between British and American English or Spanish in Spain and Spanish in Latin America.