This is a subject that comes up many times. The major doubt is who should say obrigado and who should say obrigada?

Do you know the answer? 

Obrigado  is an adjective that can mean feeling indebted to someone, owing a favour or a gesture of kindness, it can also be synonymous with obliged, grateful or acknowledged. This adjective comes from the verb obrigar.

Obrigado is also a past participle.

Obrigar – to force or to oblige (verb)

Obrigado – forced or obliged (past participle)
e.g.,

Estou-lhe obrigado – I am obliged to you

Ela ficou-lhe obrigada – she owed him

But these constructions are not common in contemporary language. So let’s take a closer look!

Obrigar

To force / to compel

Foram obrigados a mudar de casa por causa do barulho – they were forced to move house because of the noise.

Obrigação – obligation

Obrigatório – compulsory

Obrigado

Thank you

When obrigado is used as a thank you, it is being used as an adjective, and many teachers and grammar buffs, including myself, advise that the adjective should agree with the gender of the person saying it: 

Obrigado if you are a male

Obrigada if you are a female

However, it is possible to consider obrigado as an interjection, as there is nothing justifying it as an adjective when used in isolation. Interjections are invariables so it should be just  – obrigado – regardless of the gender or number of the speaking subject. 

Using obrigado as an interjection is similar to other cases where an original adjective loses its inflection and becomes an interjection.

Apoiado! = supported! (here here!)
Ótimo! = great!

So it can safely be said that it is correct for a woman to say obrigada (using an adjective that agrees in gender) or obrigado (using an interjection, which is invariable), but a man should only say thank you with obrigado, as this form is that of the singular masculine adjective and interjection. 

A nice alternative to the two is:
Obrigadinho and obrigadinha – thanks!

Same rules apply!

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