Portuguese is similar to Spanish, it is a fact. You don’t even need to be well-versed in any of these languages to realize that they are very close to each other. Just have a look at the Spanish and Portuguese version of a same text and compare. The text of your morning cereal box will do. If it wasn’t for the trademark “m” (ex: Pt. sim vs Sp. sí) and “ão” (ex: Pt. educação vs Sp. educación) that you can find in the Portuguese version, you probably wouldn’t be able to tell the difference.
Languages can be divided in families. In Europe the biggest families are the Romance, Germanic and Slavic languages. As Spanish and Portuguese are both Romance languages, they share similarities with the other Romance languages like French and Italian. They all find their origins in Latin and kept the main basic characteristics: the word order is the same, as well as the use of genders for common nouns for example. However, of all the Romance languages, Spanish is the closest to Portuguese. The most commonly used terms are almost the same, “agua”, “sol”, “comer”, “bonito”, “desculpa”, ... The conjugation system follows the same logic and declinations. However, despite all these similarities, there are enough differences and subtleties to make Portuguese and Spanish two different languages. The pronunciation is one of them. Portuguese has a lot of slurred sounds, it is a very fluid language, as opposed to Spanish, where words are more deliberate.
Besides, we have to keep in mind that there are different variants of Portuguese and Spanish. European Portuguese sounds very different from Brazilian or African Portuguese, and people in the rest of Latin and South America all speak Spanish but with a different twist. Knowing this, there are many ways of comparing Spanish to Portuguese. In South America, the gap between Spanish and Portuguese is even smaller. The 2nd person singular, “tu”, disappeared both in Spanish and Portuguese to be replaced by the 3rd person, “você” or “usted”, whereas it is only used in a formal context in Europe. Another example is the correspondant use of what is called present continuous in English, as in: “I am eating”. In Spanish (both in South America and in Europe), the translation would be “estoy comiendo”, using a similar grammatical structure. The same happens in Brazilian Portuguese: “estou comendo”. In Portugal, however, people would use the infinitive verb form and say: “estou a comer”. These are examples of the differences between Brazilian and European Portuguese that make Brazilian Portuguese closer to Spanish than European Portuguese.
Now, if speaking Spanish is a huge advantage to start learning Portuguese, it can also be a handicap. Indeed, it is sometimes hard not to mix both and end up tangled in Portuñol, a mix of Portuguese and Spanish. You can have a look at our article on false friends to have an idea of how wrong a conversation can go if you use Spanish words thinking they are the same in Portuguese.
To conclude, although Spanish is similar to Portuguese, Spanish speakers will also have to study and practice, perhaps as intensively as any other person to reach a good level in Portuguese, and speak it correctly.