Technically, there is no federal law that prevents an economy passenger from using a first-class bathroom on a domestic flight in the United States, but you are subject to the discretion and instructions of the flight crew. Airlines generally have a policy that passengers should use the lavatory in their assigned cabin to prevent "crowding" in the premium areas and to ensure the crew can perform their service without obstruction. On most domestic flights in 2026, flight attendants leverage "situational flexibility"—if the rear aisle is blocked by a beverage cart or if there is a long line at the back, they will often allow an economy passenger to use the forward lavatory if they ask politely. However, on international flights bound for the U.S., federal security regulations are much stricter, and passengers are almost always required to remain in their own cabin’s lavatory. Regardless of the cabin, the crew’s instructions are final; ignoring a direct request to return to your cabin can be classified as interfering with a flight crew, which is a serious offense.