Abstract: An cylindrical hollow anti-backflow insert that can rapidly retrofit a fire hydrant to prevent a forced liquid inflow into the fire hydrant after simple insertion of the insert into the outflow nozzles of a fire hydrant. A curved anti-backflow flap having a hinge at one end and an inflow fin at an opposite end is positioned within the insert. Outflow of water from the fire hydrant will cause the flap to rotate upwards, at which point the inflow fin protrudes into the interior of the insert sufficiently to catch any forced liquid inflow stream which will force the flap downward towards a curved flap seal. The curved flap seal reversibly engages the perimeter of the flap, thereby reversibly closing the rear end of the insert and preventing the forced inflow of liquid into the fire hydrant.