Yes, any investment income is subject to the NIIT if you are a married couple filing jointly with income over $250,000. It is assessed on the lesser of your total investment income or the amount by which your income exceeds $250,000.
Investment income includes capital gains from sales of stock and other investments, rental income, interest, and most other income that is not derived from wages, self-employment, unemployment, or retirement income. The IRS has a very thorough Q&A about the net investment income tax.
