It is legal to carry any amount of cash on an airplane inside or outside the United States, but amounts over $10,000 must be declared to U.S. Customs and Border Protection. Travelers carrying large amounts of cash should take precautions against theft.
Cash should not be packed in checked luggage and instead should be kept with a traveler at all times. Cash also should not be packed in carry-on bags that are placed in overhead bins or under seats. Money belts with metal zippers and clips can set off metal detectors at security checkpoints. Plastic belts with Velcro closures are recommended instead.