Additional Medicare tax
An extra 0.9% Medicare tax once wages and self-employment income pass $200,000 single or $250,000 married filing jointly.
Earn above a set line and Medicare takes another 0.9%. IRS Topic 560 puts that line at $200,000 if you file single or as head of household, $250,000 if you're married filing jointly, and $125,000 if you're married filing separately. Those numbers are not adjusted for inflation.
Wages, self-employment income and railroad retirement pay all go into one pile for the test. Your employer doesn't match this one. Payroll starts taking it out once your wages for the year pass $200,000, whatever your filing status, and Form 8959 squares it up when you file.
If you're self-employed, you pay it on self-employment income over the threshold. Don't expect to deduct half of it, though. The deductible half of self-employment tax leaves this piece out, per Section 164(f).
Take a single owner with $250,000 of Schedule C profit. Net earnings come to $230,875. The extra tax hits the $30,875 over $200,000: $277.88.