Interferometric Synthetic Aperture Radar (InSAR) measurements are increasingly being used to measure small amplitude tectonic deformations over large spatial scales. Residual signals are often present at these scales, and are interpreted to be noise of indeterminate origin, limiting studies of long-wavelength deformation. Here, we demonstrate the impact of bulk motion by the Earth’s tectonic plates on InSAR-derived velocity fields. The range-dependent incidence angle of the InSAR observations, coupled with plate velocities of centimeters per year, can induce long-wavelength spatial gradients of millimeters per year over hundreds of kilometers in InSAR-derived velocity fields. We show that, after applying corrections, including for the ionosphere and troposphere, plate motion represents the dominant source of long-wavelength secular velocity gradients in multi-year time series for several study areas. This signal can be accounted for using plate motion models, allowing improved detection of regional tectonic strain at continental scales.