We investigated the status of invasive exotic plants disturbing the ecosystem of the Seoul metropolitan area and examined the management of such plants. We selected our study sites based on those used in previous studies and on information in databases. All flora were classified into 57 families and 211 species; we evaluated 253 plant communities. The representative, invasive exotic species were Sicyos angulatus, Lactuca scariola, Ambrosia trifida, Ambrosia artemisiifolia, and Eupatorium rugosum. Stands of A. trifida ranged in area from 214 to 16,882 m2 and were present in riparian zones, road and forest edges, slopes, and other open habitats at all sites, covering an average of 37.87% of all study areas. In Gwangju, Ansan, and Anyang cities and in Yeoncheon-gun, A. trifida coverage was >50%; the total mean coverage was near-continuous along the Han and the South Han rivers, broken only by cliffs in some riparian zones, and anthropogenic constructions. A. trifida and S. angulatus require careful management because of extensive growth, shading, and twining. The biodiversity of native species may be conserved by physical eradication of these plants. We describe the extents and distribution patterns of these two representative invasive exotic species in the enormous urban ecosystem of Seoul.