Also see Can I use “US-American” to disambiguate “American”? If not, what can I use? and Is ‘USAers’ just an ordinary English word today? As a broad rule, United States of America is essentially never used attributively— you are a U.S. citizen, a United States citizen, or an American citizen.
By analogy with U.S. citizen, you think you can say China citizen, but Chinese citizen blocks it. U.S. citizen is different either because it predates American citizen or it means something different. e.g., it's shorthand for the legal term "citizen of the United States" (see below).
28 There is a suffix that is written only as -ize in American English and often -ise in British English (but not always, as ShreevatsaR points out in the comments). This suffix attaches to a large number of words, thus the s/z alternation shows up in a large number of words. Citizen does not have the -ize/-ise suffix.
A citizen of the United States is a legal resident who has been processed by the government as being a member of the United States. A denizen of the United States is simply someone that lives there.
Why is citizen used to describe an inhabitant of a country when the word is derived from the Latin for city (civitas) and originally meant a city dweller? Wouldn’t the nouns derived from ‘country...
etymology - Why is the inhabitant of a country called a “citizen ...
I have seen the indefinite article "a" omitted before "citizen", like in: He can enter the United States any time. He is citizen. and in similar sentences and even in legal co...