A planet’s surface conditions can significantly impact its climate and habitability. In this study, we use the 3D general circulation model ExoPlaSim to systematically vary dayside land cover on a synchronously rotating, temperate rocky exoplanet under two extreme and opposite continent configurations, in which either all of the land or all of the ocean is centred at the substellar point. We identify water vapour and sea ice as competing drivers of climate, and we isolate land-dependent regimes under which one or the other dominates. We find that the amount and configuration of the land can change the planet’s globally averaged surface temperature by up to ∼20 K, and its atmospheric water vapour content by several orders of magnitude. The most discrepant models have partial dayside land cover with opposite continent configurations. Since it will be possible in the coming years to observe the atmospheres, but not the surfaces, of such exoplanets, these land-related climate differences likely represent a limiting uncertainty in a given exoplanet’s climate, even if its atmospheric composition is known. Our results are robust to variations in atmospheric CO2 concentration, star temperature, and incident flux.