Electrochromic devices (ECDs) have broad application prospects in smart windows, low-power displays, anti-glare rearview mirrors, and so on. The electrolyte layer is one of the crucial components of ECDs acting as an interface layer between two electrodes to provide ions transport. Conventional liquid electrolytes have disadvantages of easy leakage, low safety and complex encapsulation, while inorganic solid electrolytes (ISEs) have very poor room temperature conductivity. Polymer electrolytes are composites consisting of polymeric matrices, solvents, conductive salts and curing agents, including solid polymer electrolytes (SPEs), gel polymer electrolytes (GPEs) and composite polymer electrolytes (CPEs). They are widely used in ECDs because of their excellent ionic conductivity, superior electrochemical stability, high transparency and simplicity of processing. This review provides a detailed discussion of the types, mechanisms, characteristics and preparation methods of polymer electrolytes for ECDs. The latest research progresses in the design and application of CPEs based on poly(ethylene oxide) (PEO), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVDF) and poly(vinyl alcohol) (PVA) are highlighted, and the potential applications of polymer electrolytes in the field of electrochromism are reviewed. Furthermore, the future development of polymer electrolytes in ECDs with stability, significant coloration efficiency and high ionic conductivity is discussed.