Will I owe US tax on gains when I sell my US stocks after leaving the country?

It depends on your residency at the time of sale. If you've become a nonresident alien and you sell stock from abroad, the U.S. generally does not tax your capital gains on publicly traded securities, this is a notable break for departing investors. Dividends you receive while nonresident, however, are typically withheld at 30% or a lower treaty rate. The big exceptions are U.S. real property (taxable, with FIRPTA withholding on sale) and certain situations like being a "covered expatriate." Meanwhile, your new home country will likely tax the gain under its own rules. Because the answer hinges on the exact date you become nonresident, coordinate the timing of large sales with your move, and confirm your country's treaty before assuming the U.S. exemption.