Got hit with Iowa income tax on my first paycheck for new Seattle job

As an individual you are a cash basis taxpayer. So you shouldn't have income until you actually receive money, which in your case was after you established residency in Washington. The signing bonus is the only possible source of Iowa income. The company may have applied the concept of constructive receipt to the bonus if you could have taken payment while still an Iowa resident. However, you should have been able to defer receipt, and recognition, of the income until after your relocation to Washington.

The worst that will happen is you will file a part-year resident return for Iowa and exclude the income. This should result in a refund of at least some of the tax withheld, if not all. Since Washington doesn't have an individual income tax you won't have to worry about being underpaid to them.

this is always a pain in the arse when you move states for a job. Its probably any bonus or move they provided. You will get it back, and since you have to file a partial year return anyway for iowa it won't be that much extra work.

I would call HR and ask them.