I'm Chinese.

Hi Ariel, "I am Chinese" (I'm Chinese) is correct. When "a" is in front of the nationality, it becomes an adjective which then needs to be followed by a noun. For example: a Chinese man - a Chinese woman - a Chinese car - a Chinese person. For example in this dialogue: Ariel: Hi Alex. Where are you from? Alex: Hi Ariel, I'm from Switzerland. And you? Ariel: I'm from China. What is your nationality, Alex? Alex: I'm Swiss. I'm a Swiss man. And you? Ariel: I'm Chinese. I'm a Chinese woman. I hope this was clear and easy for you to understand. Alex

I'm Chinese. But I'm a Chinese man/woman. Pay attnetion to if Chinese is an adj or a noun.

I am from China. I am a Chinese person/man/woman. I'm Chinese. I am Chinese. Take your pick, any will serve the purpose unless I say it of course because I am not from China and I am not a Chinese person.

In the year 2018, the only correct answer is "I'm Chinese." The phrase "a Chinese," in which "Chinese" is not an adjective but a singular noun, is not actually wrong--but it is outdated language. Nobody's used it in fifty years. Do not use it. It is not only outdated, it is slightly disrespectful. You might read it in an old book. It's old-fashioned. It's language used when describing national stereotypes. It would be used about someone else, not yourself. With apologies for presenting an unpleasant example of stereotyping, and extra apologies to any Koreans reading this, here is some language from 1904 by the writer Jack London: "The Korean is the perfect type of inefficiency — of utter worthlessness. The Chinese is the perfect type of industry. For sheer work no worker in the world can compare with him... The Chinese is no coward," etc.