@kmeline --
@Opus 17 is correct. My previous answer only holds if the bonus is derived from your MA employment.
If the bonus is received from your WA employer, and you receive it after becoming a resident of WA, then it is not taxable by MA. In general, you become a resident of a new state for tax purposes on the day you begin living there with the intent (as demonstrated by your actions) of making it your new main, permanent home.
Section D of this reference describes how MA treats part-year residents for tax purposes:
https://www.mass.gov/technical-information-release/tir-95-7-change-in-the-definition-of-resident-for...
WA has no state income tax.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
