A method and device performs onboard monitoring of a horizontal position of a taxiing aircraft, in which the monitoring of the horizontal position is based on separately determined height differences of the aircraft. A predetermined threshold value is compared with a first height difference calculated using repetitively determined first current height positions received by an onboard positioning device over a first duration. The predetermined threshold value is also compared with calculated second height differences calculated from two successively measured second current height positions determined by an altimeter over a second duration that is less than or equal to the first duration. A warning signal, signaling a problem with the reliability of the monitored horizontal position of the taxiing aircraft, is emitted in the event none of the second height differences is greater than the threshold value, and the first height difference is greater than the threshold value.