Additional spatial nonuniform magnetic anisotropy is induced in the basal plane of a FeBO3 single crystal using low-symmetry mechanical stresses. The effect of the nonuniform magnetic anisotropy on the magnetic state of this weak easy-plane ferromagnet is studied by a magnetooptic method. When the nonuniformly stressed FeBO3 crystal is magnetized in the basal plane near a certain preferential direction, the crystal is found to transform from a homogeneous into a spatially modulated magnetic state, which can be represented by a static spin wave in which a local ferromagnetism vector lies in the basal plane and oscillates about the average magnetization direction in the crystal.