Is there a legal limit to the amount of cash one can carry on domestic US flights?

I have not been able to get a definitive answer as to if these reported seizures are due to other illegal activity or if carrying cash over a certain dollar amount is in itself illegal.

It's not illegal but various law enforcement has a very strong motivation for invoking civil asset forfeiture -- they get to keep the money or part of it. You do not need to be even charged much less convicted of a crime.

If you are on a domestic flight in the US, there is no limit to the amount of cash or monetary instruments that you can carry. However, the Transportation Security Administration may ask a passenger who is carrying a large sum of cash to account for the money. If the Transportation Security Administration suspects that the money is related to some kind of criminal activity such as drug trafficking or money laundering, they may turn the issue over to a law enforcement agency (the Transportation Security Administration has no law enforcement powers).

I do not believe there is a legal limit, but there is a practical limit. According to Jon Moy, a million dollars weighs about 20 pounds. It appears that you can not carry much more than a million dollars onto an airplane.

I've heard that the United States government can and is seizing any amount of cash over a certain dollar amount at airports.

Customs in many countries seizes undeclared cash over a certain limit, usually around $10,000 equivalent. You can arrive at JFK with a suitcase full of Benjamins, declare them, fill out a couple of IRS forms and be on your way in 20 minutes. If you don't declare the cash you're in a bit of trouble.

Think it through though - if you got the money through legal means, it will not be difficult to account for the money if the TSA asks.

Did you just sell something very expensive? Then you should have some kind of order form or copy of the receipt.

Did someone die and leave the money to you? Then you can have a copy of the death certificate, will, etc.

Did you just take your life savings out of the bank? You'll have a receipt.

Are you just a high powered executive that gets paid loads of money, or an heir to a fortune that's always walking around with tens of thousands of dollars? Then it's easy for the authorities to verify your identity.

I'm having a hard time coming up with a scenario in which you legally acquire enough money to pique the TSA's interest (would have to be at least a couple tens of thousands) where you wouldn't be able to account for it it to their satisfaction. Tell us how you acquired the money you're worried about and I'm sure we can advise you on how to convincingly account for it.

Is there a legal limit to the amount of physical cash a US citizen can possess when flying from one US state to another? I've heard that the United States government can and is seizing any amount of cash over a certain dollar amount at airports. However, I have not been able to get a definitive answer as to if these reported seizures are due to other illegal activity or if carrying cash over a certain dollar amount is in itself illegal.

There is no limit to the amount of cash you can carry domestically or internationally.

However, traveling internationally you should declare any amount in currency or currency equivalents (like cashier's checks for example) that is above the value $10,000; this is because you need to provide the source of funds. For example, you could show a bank teller receipt.

In either case - there is no limit on the amount you can bring in or take out as long as you declare it properly.

If you cannot provide a source of funds for amounts above $10,000 - or you fail to declare that you are carrying above $10,000 and are then searched and discovered with the amount - then the government has the right to seize the entire amount (not just the amount above 10K).

In the US, this is not the job of the TSA to seize funds - this is a role for the CBP (Customs and Border Protection). The TSA is not a law enforcement agency (they cannot arrest you, for example).