The development of topography plays an important role when low-energy projectiles are used to modify the surface or analyze the properties of various materials. It can be a feature that allows one to create complex structures on the sputtered surface. It can also be a factor that limits depth resolution in ion-based depth profiling methods. In this work, we have studied the evolution of microdendrites on poly(methyl methacrylate) sputtered with a Cs 1 keV ion beam. Detailed analysis of the topography of the sputtered surface shows a sea of pillars with islands of densely packed pillars, which eventually evolve to fully formed dendrites. The development of the dendrites depends on the Cs fluence and temperature. Analysis of the sputtered surface by physicochemical methods shows that the mechanism responsible for the formation of the observed microstructures is reactive ion sputtering. It originates from the chemical reaction between the target material and primary projectile and is combined with mass transport induced by ion sputtering. The importance of chemical reaction for the formation of the described structures is shown directly by comparing the change in the surface morphology under the same dose of a nonreactive 1 keV xenon ion beam.