Alzheimer’s disease (AD) and insulin-related diabetes mellitus (DM) are chronic conditions known to affect brain structures. This study aimed to compare the brain volume changes in patients with AD, DM, and the co-occurrence of both diseases. Brain MRIs of individuals aged 18 to 90 were retrospectively analyzed. The study included four groups, each with 40 individuals: AD+DM, only DM, only AD, and a control group. Brain volumes were evaluated using the Volbrain Volumetry Report, with structures such as white and gray matter, amygdala, caudate, hippocampus, putamen, thalamus, orbitofrontal gyrus, fusiform gyrus, temporal gyrus, and ventricles assessed. The results showed no significant volumetric differences between the AD+DM group and the AD-only or DM-only groups. Moreover, no significant difference was found between the AD-only and DM-only groups, except for three parameters (p > 0.05). Both AD+DM and AD groups showed similar volumetric changes compared to the control group, while the AD group displayed more pronounced changes in specific structures, such as the amygdala, caudate, and thalamus. The DM-only group showed a significant difference only in the left caudate volume when compared to the control group. Between three patient groups, the left caudate volume showed statistical changes (p < 0.05). In conclusion, AD, DM, AD+DM cause similar brain volume changes, with the left caudate volume emerging as a key structure for clinical evaluation.