All U.S. citizens are residents of at least one state for tax purposes. Your state of residence is determined by:
You can be considered a resident of multiple states. It’s also possible to be considered a full-year resident of one state and a nonresident of another state, or a part-year resident in multiple states and nonresident in other states at the same time.
If you don't know which state you should consider your state of residence, go to the Federation of Tax Administrators webpage for State Tax Forms & Filing Options to understand the resident guidelines.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.