I was a resident in California and moved to Washington in May of 2015, employed by the same company. In 2016, I was paid a bonus by my company, but they withheld a partial-year California income tax. The bonus was paid in 2016, and I was not a resident in California during 2016. Should I have to pay California Income Tax on that bonus?
Unfortunately yes, you may have CA-source income to report.
A bonus that relates to work performance will be sourced to the location where the work related to get the bonus was performed regardless of when the bonus was paid.
So if your 2016 bonus was based on work performed in 2015 and you were a part-year resident of CA during 2015 when the work was performed, then a portion of your bonus will be considered CA-source income.
You will need to file a nonresident CA state income tax return to report the portion of your bonus that is considered CA-sourced.
You will usually get a tax credit in your home state for any taxes paid to another state but since WA does not have a state income tax, you will not be able to claim any CA state tax credit.
Here is additional information about filing in multiple states (select "see more answer" to view the entire attachment)
https://ttlc.intuit.com/replies/3300797