We report the development of a means to quantitatively evaluate the piezoelectric coefficient, d33, of room temperature ionic liquids (RTILs) and apply it to investigate how d33 varies with RTIL cation structure. The d33 quantitation method we developed also enables evaluation of the average size of pressure-induced, piezoelectrically active RTIL crystals. We evaluated d33 for RTILs composed of seven different cations and the common anion bis(trifluoromethylsulfonyl)imide (TFSI−) and found that its magnitude varies predictably with the extent of conjugation and the length of the cation’s aliphatic chain. These findings offer insights into how the constituent ion structures of RTILs can be rationally optimized to enhance piezoelectric activity.