Abstract: Assessing invasive terrestrial plant species in protected areas is of major importance, taking into consideration the role they play as key drivers in conserving biological diversity. The paper is aiming to argue the Invasive Terrestrial Plant Species (ITPS) in the Romanian protected areas with a special focus on the species Fallopia japonica in the Maramures Mountains Natural Park. Fallopia japonica, also known as Polygonum cuspidatum or Reynoutria japonica is an herbaceous perennial plant, largely occupying the riparian ecosystems and causing serious damages to native vegetation. The species is broadly regarded as one of the most invasive plant species in Europe, also listed by the World Conservation Union as one of the world’s one hundred worst plant invaders. The paper seeks to analyze the potential spread of Fallopia japonica in a protected area-Maramures Mountains Natural Park – V IUCN category as well as Natura 2000 site (SPA and SCI) integrating comprehensive statistical and field data with modern computing methods (GIS-based). Consequently, based on accurate mapping and field investigation of Fallopia japonica in the study-area, the authors were able to identify specie’s main ecological requirements and preferences, spreading conditions etc. The current research will have great contribution to undertaking further studies on invasive terrestrial plant species development, distribution potential and impact upon native habitats.