Objectives: This non-randomized and self-selected controlled intervention study aimed to examine the effects of simple nutritional guidance provided by registered dietitians on the oral function and body composition of patients in a general dental clinic in Japan. Methods: Among patients aged ≥50 years who visited a single dental clinic for regular maintenance, those who accepted and declined nutritional guidance were classified as being in the “guidance group (G group)” and “nonguidance group (nG group)”, respectively. The oral moisture content, masticatory performance, tongue/lip motor function, maximum tongue pressure, and swallowing function were assessed. Additionally, we assessed 10 body composition parameters using a body composition analyzer. Patients in the G group received monthly nutritional guidance from a registered dietician. At 3 months, changes in oral function and body composition were examined and between-group comparison in changes in dietary habits was performed. Results: A two-way analysis of variance revealed a main effect of time and an interaction between time and group for oral function only, and improvements in masticatory performance scores, maximum tongue pressure, and swallowing function were observed in the G group (n = 20). Compared with the nG group (n = 18), the G group had a significantly higher proportion of participants with a reported increase in the variety and amount of consumed food. Conclusions: Although this is the preliminary trial with a small sample and high risk biases, our findings suggest the possibility that 3 months of nutritional guidance at a general dental clinic in Japan may improve oral function.