In public transport, a request stop, flag stop, or whistle stop is a stop or station at which buses or trains, respectively, stop only on request; that is, only if there are passengers or freight to be picked up or dropped off. In this way, stops with low passenger counts can be incorporated into a route without introducing unnecessary delay. Vehicles may also save fuel by continuing through a station when there is no need to stop.

"Flag stop" airline service was historically offered by several scheduled passenger air carriers in the past into destinations with low airline passenger demand. As an example, in its June 1, 1969, worldwide system timetable, Pan American World Airways (Pan Am) had this explanation: "Flag stop: A stop will be made and traffic will be accepted only when operating conditions permit, and provided request to stop is made sufficiently in advance."

There may not always be significant savings on time if there is no one to pick up because vehicles going past a request stop may need to slow down enough to be able to stop if there are passengers waiting. Request stops may also introduce extra travel time variability and increase the need for schedule padding.

The appearance of request stops varies greatly. Many are clearly signed, but many others rely on local knowledge.

The methods by which transit vehicles are notified that there are passengers waiting to be picked up at a request stop vary by transit system and by route.

Many local bus and tram systems operate most of their stops as request stops. Buses and trams do not service stops unless there is an awaiting passenger or an onboard passenger utilizes an electric bell to signal a stop (generally by pulling a cord, or pushing a button or yellow signaling strip). Stops that are served on every trip are often called stations and placed at the terminus of a route. Such stops are often also used as timing points. In contrast, light rail and bus rapid transit services will typically stop at all stations regardless.

However, some systems use this term to distinguish between marked stops that must be hailed (as if hailing a taxicab) and marked stops where the driver will stop for any awaiting passengers (as above). This practice was common on certain Transport for London routes until 2008, with different signs distinguishing between the two sorts of stops.

Still other systems may use the term "request stop" to refer to a servicing location other than a marked bus stop. This sort of service can be found on hail and ride routes, designated portions of routes, or special late-night service. In hail and ride operations, there are few or no marked stops and passengers can request the bus be stopped at any point where the driver can safely and reasonably do so. This is common in some cities, such as Tulsa, Oklahoma, United States, where bus stops were infrequently signed before 2019. Some services operate in this way only late at night, allowing for drop-off between marked stops, thereby decreasing walking time for safety and convenience. Examples include Winnipeg Transit and New York City MTA Bus (known as Late-night Request-A-Stop).