IMO (formal) American Portuguese and Spanish are pretty close to mutually intelligible, especially in writing. There's a surprisingly consistent "system" for converting words between them and once you notice it, it's pretty easy to tell what a sentence should be in the other language, if it's even spelled differently in the first place. The grammar is also very similar. The biggest difference that gets me is how Portuguese tends to shift past tense conjugations further into the past vs Spanish.

Because Portuguese has many more vowels than Spanish and there are a lot of false friends between the two.