Photomechanical switching (photoisomerization) of molecules at a surface is found to strongly depend on molecule−molecule interactions and molecule−surface orientation. Scanning tunneling microscopy was used to image photoswitching behavior in the single-molecule limit of tetra-tert-butyl-azobenzene molecules adsorbed onto Au(111) at 30 K. Photoswitching behavior varied strongly with surface molecular island structure, and self-patterned stripes of switching and nonswitching regions were observed having ∼10 nm pitch. These findings can be summarized into photoswitching selection rules that highlight the important role played by a molecule’s nanoscale environment in determining its switching properties.