Background: Middle-aged adults may be the ideal target group for dementia-related stigma reduction interventions to encourage the utilization of services among those who may become family caregivers. Neighborhood social cohesion may diminish dementia-related stigma, particularly in terms of perceived public attitudes. The COVID-19 pandemic can further negatively impact perceived public stigma. Objective: To investigate the association between neighborhood social cohesion and dementia-related stigma during the pre- and current COVID-19 period. Methods: We employed a cross-sectional design using data from a large population-based cohort, the Tokyo Teen Cohort, in Japan. Overall, 2,469 mothers of 16-year-old adolescents self-completed a questionnaire comprising nine dementia-related stigma questions evaluating perceived public and personal attitudes. Neighborhood social cohesion was assessed using a five-item instrument. The participants were divided into two groups according to the time of assessment: prior to the pandemic’s onset (February 2019–March 2020) and during the pandemic (April 2020–July 2021). A multiple regression analysis of stigma was performed using neighborhood social cohesion as an independent variable, and caring experience, age, educational level, and working status as covariates. Results: Personal and perceived public stigma were significantly lower in participants who perceived greater neighborhood social cohesion. However, level of personal and perceived public stigma did not differ between pre- and during the pandemic period. Conclusion: Neighborhood social cohesion may be a modifiable factor for dementia-related stigma. A localized intervention to enhance social cohesion in the neighborhood community would promote the utilization of services among those who may become family caregivers.