There is not one single definition of a US person. It is defined in multiple official US documents (Securities Act of 1933, Internal Revenue Code, etc.), and each which will give a definition relevant to its context (investment, tax, etc.).
If you are here, it is very likely the you are an individual (but a US person can be an entity, a trust, etc.) and as an individual you are considered a US person if:
- You have the US citizenship, willfully or “accidentally”. Here too there is no official definition of “accidental american”, but usually: you were born in the US (and moved abroad quickly), or one or both of your parents were born in the US. This US citizenship makes you a US person, regardless of where you reside globally, and regardless of any other citizenship you would have (dual citizenship).
- You have the official status of permanent resident (green card holder), regardless of where you are currently living/travelling.
- You have spent 183 days or more in the US within the last 3 calendar years and 31 days or more within the current year. This makes you meet the “substantial presence test” and therefore qualifies you as resident for tax purpose under the IRS regulation (see Internal Revenue Code (IRC) Section 7701(b) for more details, and U.S. Treasury Regulations (26 CFR Part 301) which precises how to compute “days of presence” and defines further “exempt individuals”.
Some usefull links
- Definition of a US person in the the Internal Revenue Code, same definition that you will find on the IRS official website.
- Definition of a US person in the 1933 Securities Act
- Definition of a US person provided by the NSA