Belly’s passport is stolen from her backpack in The Summer I Turned Pretty season 3.
The theft occurs in Paris in episode 9, “Last Call.” After Belly chooses to stay in the city, her backpack is stolen at a café. The bag contains her passport and engagement ring, turning her decision to remain in Paris into an immediate practical problem.
The passport is especially important because it is an essential travel document. Belly is already dealing with uncertainty after learning that there is no room for her in the exchange program, so losing it complicates her ability to manage her unexpected new life in France.
The item is sometimes confused with the engagement ring because both are stolen together. The question asks specifically for the travel document, making “passport” the single answer. The storyline then leads Belly to a group of Parisians who encourage her to stay.