Abstract: Seismometers of the InSight probe (Interior Exploration using Seismic Investigation, Geodesy and Heat Transport) currently operating on Mars have recorded not only seismic events but also high-frequency non-seismic periodic signals that appear to have been induced by variations in the Martian environment and the hardware. Here, we report an observation of a long-period signal with a dominant period of ~20 s from Martian solar days (Sol) 800 to Sol 1,000. This 20-s signal is detected mostly at quiet nighttime — from 22:00 to 04:00 LMST (Local Mean Solar Time) — at the InSight landing site. The measurement of the particle motion suggests that this linearly polarized signal focuses on the horizontal plane with an angle of ~30° from the north. By examining the temporal variation of the signal’s amplitude and polarization angle and its times of occurrence in relation to the planet’s atmospheric data, we suggest that this 20-s signal may be relevant to wind and temperature variations on Mars. Furthermore, we study the possible influence of this 20-s signal on the noise autocorrelation and find that the stacked autocorrelograms can be quite different when the 20-s signal is present.