Abstract Mercury experiences the most intense and variable solar wind (SW) conditions in the solar system due to its close, eccentric orbit about the Sun. In addition to variation driven by coronal source and solar cycle, the SW arriving at Mercury varies periodically as the planet’s heliocentric distance changes by over 50% per orbit. We use 30 years of SW measurements near Earth and scale them to Mercury’s orbital distance to determine the importance of this ‘seasonal’ change in the SW. We find the seasonal change to be substantial, systematic, and significant, and that it dominates over other sources of variability (e.g., fast vs. slow SW) and even exceeds the relative change at Earth from ICMEs compared to the ambient SW. As a result, Mercury’s magnetosphere is expected to exhibit substantial seasonal effects.