This study assessed the effect of continuous ingestion of monosodium L-glutamate (MSG) on cognitive function and dietary score in dementia patients. Subjects/methods This was a single-blind, placebo-controlled trial involving 159 subjects with dementia residing in a hospital or nursing home. We assigned the subjects to a group that ingested MSG thrice daily (0.9 g/dose) (MSG group; n = 79) or a group that ingested NaCl thrice daily (0.26 g/dose) (Control group; n = 80). This study consisted of a 12-week intake period, followed by a 4-week follow-up period without the ingestion of MSG or NaCl. We performed physical examination, cognitive symptom tests (the Touch Panel-type Dementia Assessment Scale (TDAS) and Gottfries–Bråne– Steen Scale (GBSS)), palatability and behaviour questionnaires, and blood tests before and after the intervention and after the follow-up period. Results There were no significant differences in the TDAS and GBSS total scores between the groups before and after the intervention. However, regarding the TDAS sub-items, “the accuracy of the order of a process” did not deteriorate in the MSG group compared with that observed in the Control group (p < 0.05). At the follow-up assessment, the TDAS total scores in the MSG group showed significant improvement compared with those reported in the Control group (p < 0.05). Furthermore, there was a correlation of changes from pre-intervention to post-intervention between the TDAS and enjoyment of the meal (r = −0.299, p = 0.049). Conclusions Our results suggest that continued ingestion of MSG has an effect on cognitive function. Furthermore, the patients with improved questionnaires about palatability survey showed greater improvement in cognitive function.