Dynamically loaded structures made of thermoplastic polymers have been extensively exploited in several demanding industries. Due to the viscoelastic and thermal properties of thermoplastic polymers, self-heating is generally inevitable, especially during dynamic deformations at high frequencies. Therefore, the thermoplastic polyether ether ketone (PEEK), with its high temperature resistance and high specific strength, is a particularly ideal candidate for dynamically loaded applications. Using scanning laser Doppler vibrometry and infrared thermography, an experimental study of the vibration characteristics and the vibration-induced heating of flat-sheet PEEK specimens was carried out. The specimens were base-excited by means of a piezoelectric actuator at high frequencies in the range between 1 and 16 kHz. As a result, a maximum temperature rise of approximately 6.4 K was detected for the highest investigated excitation. A high correlation between the spatial distribution of the velocity along the beam’s axial direction and the resulting temperature increase was measured. To summarize, the occurring self-heating of PEEK due to the dissipation of vibrational energy has to be critically considered for dynamically loaded structural applications, especially areas with high displacement amplitudes, such as antinodes, which yield the highest temperature increase.