Background/Objectives: Malnutrition is a major concern among bedridden older adults because functional limitations and dependence on others may compromise adequate nutritional intake. This study aimed to identify factors associated with malnutrition among bedridden older adults in Nakhon Si Thammarat Province, Thailand. Methods: This cross-sectional analytical study used secondary data from 210 bedridden older adults. Nutritional status was assessed using the 18-item Mini Nutritional Assessment (MNA^®). Because only two participants had normal nutritional status, they were excluded from the regression analysis due to the insufficient number of observations in this category. The analytical sample therefore comprised 208 participants, and nutritional status was analyzed as a binary outcome: at risk of malnutrition or malnutrition. Demographic, functional, anthropometric, oral health, and dietary factors were examined. Group differences were assessed using the chi-square test or Fisher’s exact test for categorical variables and the Mann–Whitney U test for continuous variables. Multivariable binary logistic regression was used to identify factors independently associated with malnutrition. Results: Among the 208 participants included in the analysis, 40 (19.23%) were at risk of malnutrition and 168 (80.77%) were malnourished. These proportions refer to the analytical sample after exclusion of the two participants with normal nutritional status and do not represent the prevalence of malnutrition in the original sample. In the multivariable analysis, female sex was associated with higher odds of malnutrition (AOR = 2.302, 95% CI: 1.011–5.238, p = 0.047), while greater waist circumference was associated with lower odds of malnutrition (AOR = 0.948, 95% CI: 0.921–0.976, p < 0.001). Conclusions: Sex and waist circumference were independently associated with malnutrition among bedridden older adults with compromised nutritional status. Given the cross-sectional design, these associations should not be interpreted as causal or protective. The findings support nutritional assessment and anthropometric monitoring as part of caregiver-supported care.