Crystal Tadlock, travelling to the US from Paris, said she was saving the fruit for her onward flight to Denver.
But the apple was revealed in a random search by US border agents after her first flight landed in Minneapolis.
US Customs and Border Patrol would not comment on the case, but said all agricultural items should be declared.
The apple was handed out in a plastic Delta Air Lines bag. Ms Tadlock said she did not remove it from the bag, instead putting it in her baggage for the second part of her journey to Denver, Colorado.
When the apple was found, Ms Tadlock told the agent that she had just received it from the airline and asked whether she should throw it out or eat it.
Instead the agent handed her a £500 fine.
Ms Tadlock now has the choice of paying the fine or fighting the penalty in court. She told the Denver-based broadcaster KDVR she wanted to take the case to court.
"It's really unfortunate someone has to go through that and be treated like a criminal over a piece of fruit," Ms Tadlock said.
A Delta Air Lines spokesperson refused to comment specifically on the incident.
However, the company issued a statement saying that "we encourage our customers to follow US Customs and Border Protection protocols."
More about: Delta
