Swellable poly(ionic liquid)s (SPILs) are ionic polymers featured ionic liquid repeating units in the backbone, exhibiting unique swelling ability to many solvents. SPILs can absorb solvents into a swollen state, accompanied by obvious weight/volume increase, color change, and the formation of internal porous structure. Their swelling ability depends on the compatibility of polymer chains with solvent molecules, and dissociation of ionic groups in solvents. Compared to non-swellable poly(ionic liquid)s, the good affinity for solvents endows SPILs with distinctive physicochemical properties, and these properties make SPILs particularly promising materials for various applications. This review offers a thorough understanding of the relationship between structure with swelling ability and describes the primary applications of SPILs. After introducing the chemical structures of ionic liquid monomers and crosslinkers as SPIL precursors, the synthetic strategies for SPILs with high swelling ability are discussed. Besides, the effects of different cations, anions and crosslinkers on the swelling ability are summarized. Applications of SPILs as catalysts, actuators, sensors, absorbents and separation membranes are highlighted. And challenges and opportunities in future research on this topic are prospected. This review aims to illustrate the swelling behavior of SPILs, also to help design SPILs with ideal swelling ability in target solvents to accommodate the growing practical needs.