The solubility of poly(methyl methacrylate) (PMMA) in 1-alkyl-3-methylimidazolium ionic liquids (ILs) with different anionic structures has been explored. Nearly monodisperse PMMA-grafted silica nanoparticles (PMMA-g-NPs) were used as a measurement probe for evaluating the PMMA solubility in ILs. The hydrodynamic radius (Rh) of PMMA-g-NPs was measured in the ILs by dynamic light scattering (DLS). Changes in Rh and colloidal stability, that is, the PMMA-solubility change in the ILs, were observed depending on the ionic structures of the ILs. The solubility was mainly affected by the anionic structures of the ILs rather than by the alkyl chain length of the cationic structure. Solvent parameters, including Lewis basicity, solubility parameters, and a hydrophobicity parameter, were used to discuss the change in the PMMA solubility in ILs with different ionic structures. By considering the PMMA solubility in the ILs using these parameters, it was found that there is a good correlation between the PMMA solubility and the hydrophobicity parameter of the anions. Although the change in the PMMA solubility with different cationic structures was not remarkable, the hydrophobicity of the cations also played a role in the solvation of PMMA by providing a low-polarity environment adequate to dissolve PMMA.