Portuguese speaking people can easily understand Spanish (Castillian) but Spanish speaking people have to learn to understand Portuguese, although it is not that difficult for a Spanish speaking person to learn Portuguese.
Many of the infinitive form of the nouns and verbs are almost identical, although shorter in Portuguese, and gender constructs are the same. However, there are differences in phonetic sounds, grammar, verb conjugation (Portugues is much more irregular), length of words, and contracted words that would mystify a spanish speaker. For example:
Portuguese: não (pronounced with sounds that do not exist in Spanish) = no
Portuguese: no (contraction of "em o") = in the
Spanish: no = no
Spanish: en la = in the
The Portugues person can understand that "en la" in Spanish is the equivalent of "em o" without any stretch of the imagination, but a Spanish speaking person could not understand that "no" is the equivalent to "en la".
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On a side note, one of the funniest things I ever saw was a Catalan speaking man trying to buy pizza for his family in a Portuguese mall in Porto. There was absolutely not mutual intelligiblity between those languages even though the distance between their respective regions is only about as far as San Francisco is to Denver.
I think knowing how much pride both Portuguese and Catalan speaking people have in their respective languages enhanced my ability to find humor in the situation. Both groups have spent literally over a thousand years resisting the encroachment of Castillian language and culture.