Self-organization is an interesting route to the fabrication of nanostructured magnetic materials. Here we show that, near room temperature, an ultrathin Fe film deposited on a suitable MnAs template spontaneously breaks into a “lateral” superlattice of magnetic stripes. The magnetic superstructure originates from the temperature-dependent morphological change in the substrate: an epitaxially grown MnAs/GaAs(001) film, whose groove-ridge structure was investigated by scanning tunneling microscopy. Owing to the stray magnetic fields produced by the underlying MnAs template, the Fe stripe domains have opposite magnetizations, and behave essentially as independent magnetic entities because of strong stripe edge pinning. This is shown dramatically in terms of a split microwave resonance that can be controlled with an external magnetic field, as proved by Brillouin light-scattering data and analysis of the Fe spin-wave frequencies. Additionally, the potential for device applications of such lateral magnetic superlattices, displaying an “inverse” exchange-spring behavior, is discussed.