Someone from Taiwan is Taiwanese. Someone from Brazil is Brazilian. Someone from Israel is Israeli. What is someone from The United States called? Is someone from The United States called United Statesian?
The standard and official term is "American". This is controversial because the entire New World is also known as "The Americas" and, for many of its residents, "American" ("americano") applies to their entirety; in Spanish, the adjective used for people from the U.S. is "estadounidense". (I learned "norteamericano" in Spanish class in my youth, but that's still a problem since even North America isn't just the United States, and "estadounidense" is what I've mostly come across in recent years.) I'm not making an argument for or against "American" here, and this isn't the place for that. I'm just giving you some context for your awareness.
There are no alternative, generally accepted demonyms for the United States in English. Therefore, if you choose to be sensitive about this issue, you'd need to write "people from the United States", "U.S. citizens", "U.S. residents", etc. This is what I usually do when writing in forums like this with international readership.
Some proposed alternatives are covered by Wikipedia at https://en.wikipedia.org/wiki/Names_for_United_States_citizens#Alternative_terms. I'd come across Frank Lloyd Wright's "Usonian" before.
Although there are some who deplore it, since it can truly be applied to people from two continents, not just one country, the term used is “American”. Every country on the two continents other than the United States has a nationality adjective based on the country name (e.g, Argentine, Brazilian, Costa Rican, Peruvian, etc.).
As mentioned in the other answers, the standard term is "American", although it applies to inhabitants of the whole continent.
Besides the excellent alternatives listed in Green Grasso Holm's answer, there is also the colloquial term "Yankee". However, while outside the US it is used to define all U.S. citizens, in the United States it is mostly used as a derisive term by Southerners to define Northerners.
German often uses the adjective US-amerikanisch for anything associated with the United States except for people, who are known in German-speaking countries, and practically everywhere, as Amerikaner (also a big, iced cookie for some reason).
One could easily adopt this nomenclature:
USAmerican Olympic team, USAmerican foreign policy, USAmerican regional cuisine
but I doubt that people would readily identify themselves as a USAmerican, since although it looks cool in print with the USA in front, it would be rather cumbersome in speech.