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We report a systematic rheological analysis of perfluorosulfonic acid (PFSA) ionomer monolayers at the air/water interface. Equipped with a custom-designed double wall knife-edge interfacial rheometer, we measure both the linear and nonlinear viscoelasticity of various PFSA ionomer monolayers. Based on rheological measurements along with other static measurements, we find that the rheological properties of the PFSA ionomer monolayers mainly originate from the electrostatic interaction of negatively charged groups (SO3−) rather than direct interactions between backbones, as is typical of other polymeric monolayers. Although the rheological properties mainly come from the SO3− groups, equivalent weight (EW) and length of the side chain affect the rheological properties as well: (1) surface activity increases with EW, thereby a larger EW tends to have stiffer interfaces; (2) PFSA ionomer with longer side chains also has larger shear moduli at the same surface pressure (Π), presumably because the longer side chain has larger configurational entropy, thus leading to better closed packing. Moreover, it is found from Π dependent measurements that both the elastic and viscous moduli exhibit unusually weak dependence on Π compared with typical polymeric and surfactant monolayers. The weak Π dependence can be explained in terms of charged colloids with long-range repulsive interactions, as they are susceptible to shear stress but resistive to compressional stress. This rheology of ionomers and their underlying morphology at the interface are expected to provide useful information for various membrane applications where ultrathin membranes can be helpful, such as ion exchange membranes and fuel cell/flow battery membranes. [ABSTRACT FROM AUTHOR]