Compact crystalline cellulose was structurally transformed into a loose regenerated form using treatment with tetraalkylammonium halides in dimethyl sulfoxide (TAAHs/DMSO) and gel formation using water. Swelling of cellulose was affected by the TAAHs content in DMSO and the anion and/or cation type. The cellulose swelling ratio of TAAHs/DMSO was in the order of F− > Cl− > Br− ≈ I− due to differences between hydrogen bond basicity (β) values of TAAHs/DMSO. The swelling ratio varied with the cation size of tetraalkylammonium chlorides (TAACs). For tetraalkylammonium fluorides (TAAFs), swelling ratios were all high and independent of cation size. Decrystallization of cellulose varied with the TAAHs anion and/or cation type in a pattern similar to cellulose swelling ratios. Structural transformation of cellulose, which is due to disruption of hydrogen bonds and van der Waals and hydrophobic interactions in cellulose chains and sheets by anions and cations of TAAHs, was confirmed based on FT-IR, XRD, FE-SEM, TGA, and enzymatic hydrolysis reactions.