You do not need an ITIN if you would be filing as married filing separately. In that case, you only list your own income and deductions. Married filing separately will usually cause you to owe more tax compared to married filing jointly, because many deductions and credits are reduced or disallowed. However, if you file jointly, you would have to include all of your spouse’s worldwide income even though your spouse is not a US citizen. You would get a credit or deduction for foreign income taxes that you and your spouse paid. Only you and your spouse can figure out whether it is better for you to file jointly or separately next year by preparing different test returns that test each scenario.
If you will have lived outside of the US for at least 330 days this year, you will probably qualify for the foreign earned income exclusion. That means that you do not even have to pay US tax on income that you receive in a foreign country as long as that income is “earned” from working. (Unearned income, such as passively receive rent, or prizes or interest and dividends, is still subject to US tax.)
Normally, you only apply for an ITIN for your spouse by attaching the application to your first married filing jointly tax return, because that is what gives them a financial connection to the United States. There are a few exceptions that would allow you to apply for an ITIN early, you would have to check the instructions on the form W7 or the IRS website to see if they apply to you.
