When flying internationally, amounts exceeding $10,000 must be declared to customs. Otherwise, there is no established legal limit to the amount of cash a traveler can carry onto a plane.
The U.S. Transportation Security Administration advises travelers to speak to a screening supervisor and request private screening if they are carrying anything valuable. Carrying large amounts of cash is legal, but risky. A traveler can keep the cash close to his body for safety, preferably in multiple sections.
A supply of smaller bills can be kept handy to avoid drawing attention to larger amounts. Travelers can deter pickpockets by carrying their money in a sturdy theft-proof bag. Large amounts of cash must not be transported in checked luggage.