The influence of Jahn-Teller (JT) distortion on the electrochemical behavior of LiMn2O4 is not fully understood nowadays. This paper concerns the role that structural defects play in reducing the magnitude of JT distortion. Structural defects were induced in micrometric and nanometric LiMn2O4 using Co-60 gamma radiation. Techniques such as X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and magic angle spinning spectroscopy (Li-7 MAS NMR) were used to detect and investigate the defects created by Co-60 gamma irradiation in micrometric and nanometric LiMn2O4. Differential scanning calorimetry (DSC) was used to study the effect on the JT distortion enthalpy. Spectroscopic and microstructural modifications of LiMn2O4 produced by the action of radiation are shown for the first time. The results revealed that partial and total edge dislocations are generated after the oxides are irradiated. In the formed dislocations, manganese and lithium ions modified their surroundings and coordination, while the atomic positions of oxygen atoms changed in the fcc structure. These modifications gave rise to a decrease in the magnitude of JT distortion in the micrometric oxides, while no distortion was detected in the nanometric oxides. Because of the defects in LiMn2O4 caused by Co-60 gamma irradiation, a minor distortion in the cell parameter (c/a) should be expected when this material is used as a cathode in lithium-ion batteries.