I'm a non-US citizen, and not residing in US, who uses an online brokerage firm to trade US stocks and options. Are my capital gains (short term and long term) and dividends subject to taxes? If yes, what are those taxes?
Generally speaking, if you are a long term investor in non-real estate securities and spend no time in the US, your capital gains are not taxable.
If the short term gains appear to be from trading and not investing, they may be taxable.
That said, you're still required to pay the capital gains/income taxes on all your income, home and abroad, in your home country.
Any tax experts feel free to edit for accuracy.
A quick update for people finding this thread through Google. With the help of a few awesome Bogleheads, I compiled all the relevant research done into two Wiki articles:
This includes comparing US to Irish domiciled ETFs, how to calculate tax withholding leakage and estate tax concerns.
Hope you find this useful.