Proof-of-payment (POP) or proof-of-fare (POF) is an honor-based fare collection system used on many public transportation systems. Instead of checking each passenger as they enter a fare control zone, passengers are required to carry a ticket, pass or a transit smartcard to prove that they have paid the valid fare. Fares are enforced via random spot-checks by inspectors such as conductors or enforcement officers, to ensure that passengers have paid their fares and are not committing 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 immediately 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.

This 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. It may be 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. However, in some countries it is common even on systems with very high passenger volume. Proof-of-payment is usually applied on one-person operated rail and road vehicles as well as on automatically operated rail lines.

The honor system 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 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 serving the stadium. Direct fare collection methods may also be used at major hubs in systems that otherwise use POP. An example of this is the Tower City station on Cleveland's RTA Rapid Transit Red Line, which uses faregates.

Travel without a valid ticket is not usually considered a criminal offense, but a penalty fare or a fine can be charged.