Is it fine to describe people of the USA as "US people"?
For instance: "the US people display different cultures and traditions."
What I want to ask is that can I use the word "US" as an adjective to describe the noun "people"?
Is it fine to describe people of the USA as "US people"?
For instance: "the US people display different cultures and traditions."
What I want to ask is that can I use the word "US" as an adjective to describe the noun "people"?
This usage is awkward and uncommon, but for reasons of idiom rather than grammar. "American people" is far more common.
To get a few technicalities out of the way: "US" is not technically a word, but an initialism. This shouldn't matter, though, since we use such abbreviations as if they're words ("I got a call from some FBI agents"), and they can definitely fill syntactic roles. It's also worth noting that various style guides can make different recommendations about the styling of initialisms, and some might want "U.S." rather than "US"; this also helps avoid confusion with the pronoun us.
You could use similar initialisms to modify similar nouns without idiomatic difficulty: "The NASA scientists had a breakthrough." You could even use "people" in this construction with groups that are not political or demographic, though this usage would be informal: "The CEO was on the phone with the Google people for an hour." This would be equivalent in meaning and tone to "those folks from Google."
There are also many examples of "U.S." used as an adjective to modify many nouns, just not "people": "U.S. Treasury," "U.S. interests," "U.S. citizens."
The difficulty is that many proper nouns for countries, people groups, and ethnicities have associated adjective forms. These often apply to people belonging to that group—"demonyms," words that refer to a person or people from a place. America -> American, China -> Chinese, Midwest -> Midwestern, Slav -> Slavic. When these forms exist, they insist on being used. "The America people," "the Midwest people," etc. are not idiomatic. However, some groups have no such form, and these use the same form for noun and adjective: "The Inuit elders," "the Uyghur population," "the Easter Island residents." (I note that most of these examples are demographic groups or geographical entities below the level of sovereign nations; most countries seem to have adjectival forms. Also, some groups have adjectival forms that are not always used: Iroquoi and Iroquoian can both be found in dictionaries as adjectives meaning "of or relating to the Iroquois people."
... And here's where attempts to explain break down. I was going to conclude by claiming that when an demonym exists it's always preferred. But the fact is, "U.S." is regularly used as an adjective. Some phrasal constructions feature it; "U.S. interests" has supplanted "American interests" since the 1970s. In most cases that a country has an initialism that is widely used in English, that initialism also serves as an adjectival. This Wikipedia table lists U.S. as an adjectival for the United States of America along with American, and UK along with British. When the USSR existed, usages like "a USSR spy plane" were common.
N-grams are a notoriously inexact way to interrogate usage, but if we graph "American people" along with "the U.S. people" (including the to filter out "us people," using the pronoun us) and "people of the U.S.", "American people" by far outstrips the other two. "British interests" dwarfs "UK interests."
Language isn't governed by rules, it's described. It is safe to say that demonyms are generally favored over initialisms as adjectives, except in a few specific phrases and usages like "U.S. interests." And the remarkable lack of examples of "U.S. people" makes it safe to say that this construction simply isn't idiomatic.
Using the phrase "US people" is OK, but not ideal. "People" is the problem, not using "US" as an adjective. U.S. (many style guides prefer periods in this case to avoid confusion with the word "us") clearly defines the country (the United States of America), and not just a region or continent or landmass.
"U.S. residents" (citizens, voters, workers, taxpayers) is often used. But "people" doesn't give enough information. Are they people "in" the country? Residing in the country? Visiting the country? In favor of the country? Born in the country?
"US people" is uncommon and not idiomatic. It's also vague; nearly anything more specific will work better: US citizens, inhabitants of the United States, US natives, the US population, etc. The closest use to "US people" that you are likely to see is "US persons" (or more frequently, "non-US persons"). That usage shows up in defense and similar, where it has a specific definition, depending on context.
As @AndyBonner suggests, you will want to check your preferred style guide for how "US"/"U.S." should be styled.
We can use US as an adjective. We usually use US citizen, US embassy, US dollar, US navy etc. But we don't normally use US people. We say 'American people'.
(The British- people from Britain)
You ask whether "US people" is a valid phrase. There are at least two different senses of the word "people", depending on whether they are considered individually or as a collective group. The first sense would be used as a plural noun, while the second would be used as a singular noun. The example you give, "the US people display different cultures and traditions", straddles both meanings. Temporarily putting aside "US", which complicates matters, one might say, for instance:
The second sentence is formal.
To describe people from the US rather than from France, one could use "American", as has been noted above:
In the first sentence, one could also use "US" (or "U.S."), but in a position other than before the noun:
For the second sentence, one could write:
However, this sounds more awkward than "The American people..."