"Japanese" is definitely a noun, so it's not grammatically incorrect to say "I am a japanese", but it does sound weird. This is probably related to idiomaticnes

"Japanese" is definitely a noun, so it's not grammatically incorrect to say "I am a japanese", but it does sound weird. This is probably related to idiomaticness. In the US, usage of words like "an american", "a canadian" and "a mexican" as nouns generally "sound" ok, as far as popular speech goes, whereas "a british", "a french" and "a spanish", for example, sound strange despite being grammatically correct ("A german" and "a korean" sound a bit weird too, FWIW). As far as I can tell, countrymanship nouns are mostly used with definite articles (e.g. "The japanese participated in WWII"). In other contexts, the rules are mostly based on idiom popularity, if not completely arbitrary. "New yorker" sounds perfectly fine as a noun, but "a san franciscan" is something not many people would ever utter, even in adjective form. Similar discrepancies can be seen in other noun groups, for example "a beer" is fine, "the wine" is fine, but "a wine" sounds weird. For the cases where usage of the word as a noun isn't considered idiomatic, people usually say "a korean person" or "a german guy" or "british woman", etc.