The additional Medicare tax is a 0.9% surcharge on earned income above a threshold: $200,000 for single filers, $250,000 for married couples filing jointly, and $125,000 for married people filing separately. It stacks on the regular 1.45% Medicare tax, so wages above the line are taxed at 2.35% for Medicare. Unlike the Social Security tax, there is no upper cap. These thresholds are fixed by law and do not adjust for inflation.
Employers must start withholding it once your wages with them pass $200,000 in a year, no matter your filing status. That creates a mismatch for some households. A married couple where each spouse earns $180,000 has $360,000 of combined wages, $110,000 over the joint threshold, but neither employer withheld anything because neither salary crossed $200,000. The $990 owed shows up on the tax return.
The reverse happens too: a single earner at $230,000 married to someone with no income has withholding on $30,000 of wages but owes it only on the amount over $250,000, which is zero, so the withholding comes back as a credit.
The amounts are small relative to income, but the common mistake is being surprised by them. Bonuses and RSU vests count as wages here, so a big equity year can push a household over the line for the first time. A quick check against the thresholds in the fall is enough to plan for it.