Dietary assessment of community-dwelling Japanese older adults who live alone using Dietary Reference Intakes (DRIs) is limited. The present study aimed to estimate the prevalence of inadequate nutrient intake among Japanese community-dwelling older adults by sex and young-old and old-old population groups to identify the most vulnerable groups that need support. A cross-sectional survey was conducted from October 2012 to October 2013 with community-dwelling adults aged 65 y or older who lived alone. Participants were drawn from six cities in four Japanese prefectures. We used two days of dietary survey data to estimate participants' usual intake. The proportion of inadequate intake was assessed using the Estimated Average Requirement (EAR) cut-point method, Tolerable Upper Intake Level, and the tentative dietary goal for preventing life-style related diseases (DG) based on DRIs for Japanese, 2015. Data for 494 participants (162 male, 332 female) over 988 d were examined to assess nutrient intake. We calculated the proportion of inadequate intake between males and females and between young-old and old-old population groups (65-74 y and ≥75 y) using chi-square or Fisher's exact tests. For six nutrients, the estimated proportion of participants with a usual intake below EAR was higher in males than females. In addition, a higher estimated proportion of female participants was within the DG range for the percentage of energy from protein, fat, and carbohydrates, and had a usual intake of two nutrients above DG than males. Our findings showed that among Japanese older adults, males were more vulnerable in terms of inadequate nutrient intake compared with females.