Agents from Customs and Border Protection say a currency detection dog alerted them to a woman heading to a flight bound for Montego Bay, Jamaica.
The woman, a U.S. citizen, allegedly admitted to possessing $5,000, but agents only found about $1,800 on her.
However, when they checked her luggage, they found another $41,500 concealed in a suitcase liner.
The cash was seized and the woman was released.
There is no limit to how much currency travelers can import or export; however they are required to declare amounts exceeding $10,000 in U.S. dollars or equivalent foreign currency.
"Travelers who deliberately refuse to comply with currency reporting requirements run the risk of losing their currency, and potentially face criminal charges," said Allan Martocci, CBP Port Director for the Area Port of Philadelphia in a statement. "The easiest way to hold on to your currency is to report it. It's that simple."
For more information about rules governing travel to and from the United States, visit the website of Customs and Border Protection.