A series of dicationic ionic liquids (DILs) with two singly charged imidazolium rings linked by different alkyl chain lengths (C-4, C-6, and C-8) were first synthesized. The relaxation and entanglement of poly(methyl methacrylate) (PMMA) chains in both the DILs and conventional monocationic ionic liquids (MILs) with the same anion bis[(trifluoromethyl)sulfonyl]imide ([TFSI](-)) were investigated by rheological and dynamic light scattering (DLS) measurements. PMMA in DILs exhibits lower critical entanglement concentration, slower relaxation behavior, and a denser entanglement network than in their MIL counterparts, and these differences become more pronounced as the alkyl chain length decreases. The double imidazolium ring structure that provides interaction sites with PMMA chains plays a dominant role in the relaxation and entanglement of the PMMA/DIL system. As a result, the PMMA/DILs show higher modulus, greater viscosity, and better gel recovery than the PMMA/MILs at the same volume fraction. In addition, a stronger dependence on alkyl chain length is also found in PMMA/DILs. With the increase of alkyl chain length, the relaxation of PMMA becomes faster and the entanglement molecular weight increases gradually, indicating the released mobility of polymer chains. Combined with the results of DLS, it is further confirmed that the greater degree of entanglement in PMMA/DILs mainly comes from the contribution of cohesional entanglement in the system.