Nematicity is ubiquitous in the electronic phases of iron-based superconductors. The order parameter that characterizes the nematic phase has been investigated in momentum space, but its real-space arrangement remains largely unexplored. We use linear dichroism (LD) in a low-temperature laser–photoemission electron microscope to map out the nematic order parameter of nonmagentic FeSe and antiferromagnetic BaFe2(As0.87P0.13)2. In contrast to structural domains, which have atomic-scale domain walls, the LD patterns in both materials show peculiar sinusoidal waves of electronic nematicity with wavelengths more than 1000 times as long as the unit cell. Our findings put strong constraints on the theoretical investigation of electronic nematicity.