My employer has undocumented workers on his payroll and is not withholding taxes from their checks. What can I do to report him?

Of course, they will want your name and information when you report, if not, it is very possible they will not investigate unless they have real beleif it is happening. Also you may want to go ahead and get a new job first, since if there is a lot working there, a major raid may close the business.

But contact the local office for immigration in your area. "ICE" give them all

https://help.cbp.gov/app/forms/illegal_act/


Or call (866) 347-2423

As Chuck advises, reporting the employer to ICE is one avenue. Your local state Dept of Labor is another. The IRS is a third.

But are you prepared to lose your job? While there are whistle blower laws to protect you, that won't matter if the employer is forced out of business because of fines or jail.

Be sure, of course, that you know whereof you speak first.
How do you define undocumented? It has different meanings. Never provided the required IDs when hired? Known to be illegal aliens?
There is nothing illegal about not withholding taxes as long as the person is working on a contract basis, and meets the IRS rules for being one.

Everything FR Chuck, Joy and Scott noted are all valid considerations. I truly endorse all of their cautions about losing your job. THAT you CAN count on happening if you turn your employer in, irrespective to the protections offered to whistle blowers.