A GPS jammer is a jammer for the signals of the Global Positioning System (GPS), a satellite-based system for worldwide positioning.

Due to the relatively low transmission power of the satellites and the orbit at an altitude of 20,200 kilometers above the earth's surface, the GPS signals can only be received with extremely low field strength. Therefore, even interference signals with low field strength can prevent the reception of the GPS satellites. An additional variant (e.g., noise modulation) of an interfering transmitter can temporarily or even completely prevent GPS reception.

In contrast to a GPS jammer, the much more complex GPS spoofing not only interferes with the satellite signal but also sends manipulated GPS data.

The term "Navigation Warfare" includes strategies to use GPS against an enemy and to exclude him from using the system. One scenario is based on dropping a large number of small jammers over a crisis area and thus disrupting the reception of the signal on the ground.

To minimize the effect of GPS jamming, GPS antennas with a pronounced directional characteristic can be used for certain applications (e.g., in aviation). These antennas only receive signals from the desired direction - and thus do not receive the interference signals from GPS jammers on the ground.