I am a US citizen, married to a non-US Person who has never a greencard nor lived in the US. We both live together outside the USA. We have separate foreign (non-US) bank accounts and have transferred substantial amounts from my account to hers and vice-versa. Are these transfers considered "gifts" by the IRS and, if so, subject to a "gift tax"?
No, but...
If your spouse is a non-US person for US tax laws, then any gifts she gives you are treated according to whatever tax laws she is covered by.
If you are a US person for US tax purposes, then spousal gifts are not subject to gift tax reporting requirements. But, gifts from foreign persons to a US person must be reported on form 3520 if the amount of total gifts is more than $100,000 per year. This is only a reporting requirement with no imposition of tax. The IRS wants to keep track of large international money transfers to make sure there is no money laundering or work income disguised as gifts.
See link for more.
I *THINK* if there are gifts over $168,000 (2016 amount) in a year, then a Gift Tax return needs to be filed.
<a rel="nofollow" target="_blank" href="https://www2.deloitte.com/content/dam/Deloitte/us/Documents/Tax/us-tax-us-estate-and-gift-tax-rules-for-resident-and-nonresident-aliens.pdf#page=7">https://www2.deloitte.com/content/dam/Deloitte/us/Documents/Tax/us-tax-us-estate-and-gift-tax-rules-for-resident-and-nonresident-aliens.pdf#page=7</a>
However, you would also need to determine if it truly is a "Gift", spousal support, or something else.