This dataset provides an activity-based synthetic population (Seoul5-ABSP) representing approximately 1.8 million individuals residing in five districts of Southwest Seoul, Republic of Korea (Geumcheon, Guro, Gwanak, Yeongdeungpo, and Dongjak). The dataset was generated through an integrated population synthesis pipeline that combines registered population statistics, living population data, household composition distributions, and mobility-derived temporal patterns. Individual-level records include demographic attributes, household affiliations, and spatio-temporal activity profiles at multiple daily time slots. The archived files include the core synthetic population, household- and person-level datasets, time–activity panels, intermediate probability tables, and validation summaries. The dataset is intended to support reproducible research in urban demography, population health, transportation modeling, and digital twin applications.