I am bringing more than $10,000 into the US, but this money belongs to someone else and is not my own. Do I have to report this to the IRS and/or pay tax on it?

[Critter]

If it is not income then you do not report it on an income tax return.

[BMcCalpin]

You may or may not be aware of it, but you certainly need to report this to the Customs and Border Protection people when you enter the country - see https://help.cbp.gov/app/answers/detail/a_id/195/~/currency-and-monetary-instruments---amount-that-c...