This study aims to classify rural areas based on their functional linkages with cities, moving beyond the administrative rural–urban dichotomy established during the population-growth era. Using the OECD Functional Urban Area (FUA) methodology, rural areas across Korea are categorized into three types: rural areas within FUAs (27.7%), rural areas adjacent to FUAs (20.1%), and rural areas distant from FUAs (52.2%). The analysis confirms that 67.4% of population-decline areas are located in rural areas distant from FUAs, indicating that weak accessibility and limited urban linkages are closely associated with demographic and economic decline. These findings suggest that, first, the spatial unit of rural policy needs to be expanded from administrative districts to functional areas, and second, tailored development strategies are required for each rural type. This study contributes to redefining rural areas and provides a foundational framework for national land and rural spatial planning in the context of urban–rural functional linkages.