Ce livre vous donnera une idée claire du mode de vie étasunien.

Are “étasunien” and “étatsunien” both considered correct French?

Robert dictionary only considers étasunien without T in the middle as correct.

On a sidenote, French people will almost always say américain instead. Way more natural.
The same way as an American would say “I am American“ rather than “I am a United States American”.

Thank you. There are a lot of questions towards the end of the French Fluency Fast Track where the cloze word is “étasunien” or “étatsunien” or a plural / gender variation.