(1) Background: Continuous participation in a comprehensive dementia prevention program is important for community-dwelling older adults during the coronavirus disease (COVID-19) pandemic, as limitations on their communities and social participation have increased and participation in daily tasks has decreased. These factors can negatively affect their cognitive function and symptoms of depression. This study aimed to introduce an evidence-based online dementia prevention program in the South Korean context and to identify its effect on cognitive function and symptoms of depression in community-dwelling older adults during the COVID-19 pandemic. (2) Methods: One hundred and one community-dwelling older adults without dementia participated in twelve sessions of an online dementia prevention program designed by occupational therapists. Cognitive function and symptoms of depression were assessed before and after the program. Cognitive function was tested using the Cognitive Impairment Screening Test and symptoms of depression were assessed using the Korean version of the Short Geriatric Depression Scale. The participants’ opinions were gathered using open-ended questions. (3) Results: After the program, according to the raw score, orientation was maintained and attention, visuospatial function, executive function, memory, and language function increased. The memory and total cognitive score was improved significantly. Symptoms of depression significantly decreased. The program’s benefits according to the participants were participation in new activities, boredom reduction, online communication, and reminiscence. (4) Conclusions: An online dementia prevention program is effective in maintaining and increasing cognitive function and preventing depression in community-dwelling older adults. An online dementia prevention program is a useful method in providing opportunities to participate in cognitive training and continuous daily activities during the COVID-19 pandemic.