Proof-of-payment or POP is an honor-based fare collection approach used on many public transportation systems. Instead of checking each passenger as they enter a fare control zone, proof-of-payment requires that each passenger carry a ticket or pass proving that they have paid the fare. Ticket controllers or conductors make periodic checks to deter fare evasion.

On many systems, a passenger can purchase a single-use ticket or multi-use pass at any time in advance, but must insert the ticket or pass into a validation machine before use. Validation machines in stations or on board vehicles time stamp the ticket. The ticket is then valid for some period of time after the stamped time.

Advantages of proof-of-payment include lower labor costs for fare collection, simpler station design, easier access for mobility-impaired passengers, easier access for those carrying packages or in case of an emergency, and a more open feel for passengers. On buses, proof-of-payment saves drivers the time needed to collect fares, and makes it possible for all doors to be used for boarding. Validated tickets can double as transfers between lines.

Disadvantages include potentially higher rate of fare evasion, reduced security on station platforms when no barrier is used, increased potential of racial profiling and other unequal enforcement as "likely fare evaders" are targeted, and regularly exposing passengers to unpleasant confrontational situations when a rider without the proper proof is detained and removed from the vehicle. Visitors unfamiliar with a system's validation requirements who innocently misunderstand the rules are especially likely to get into trouble.

The proof-of-payment method is implemented when the transit authority believes it will lose less money to the resultant fare evasion than it would cost to install and maintain a more direct collection method. Generally it is used in systems whose passenger volume and density are not very high most of the time—as passenger volumes increase, more-direct collection methods become more profitable. POP can be complemented with a more direct collection approach where this would be feasible—a transit authority utilizing POP will usually post fare inspectors, sometimes armed as a police force, to man entrances to stations on a discretionary basis when a high volume of passengers is be expected. For example, transit users leaving a stadium immediately following a major concert or sporting event will likely have to buy a ticket from an attendant (or show proof of payment) to gain access to the station(s) servicing the stadium.

Proof-of-payment is popular in Germany, where it was widely introduced during the labor shortages resulting from the Economic Miracle of the 1960s. It has also been adopted in Eastern Europe and Canada and has made some inroads in newer systems in the United States. The first use of the term "POP" or "Proof of Payment" on a rail line in North America is believed to have been in Edmonton in 1980.

Some urban mass-transit systems in the United States have gone the other way, however, and are installing fare barriers that are harder to evade. One example is the Massachusetts Bay Transportation Authority, which in 2006 installed subway fare gates with automatic doors instead of a traditional turnstile, making it harder to jump over or duck under the mechanism. However they are enacting a quasi-proof-of-payment system for the surface light-rail service. Riders paying with cash must pay at the front door and will be issued a receipt as POP. Holders of prepaid fare media will be able to validate their tickets beforehand at some stations and have a receipt issued. At other stations, personnel with handheld scanners will deduct value from cards or check for passes, allowing these riders to board at the unattended doors. If no inspectors are on duty those with stored value media are expected to use the farebox on board while those with passes can board at the unattended doors.