Invasive plants are recognised as a major threat to biodiversity. Although they are well-established in natural areas, the supposed negative impacts of invasive plants upon communities and ecosystems have so far been poorly investigated in urban areas, where invasions are a main issue for ecologists and for urban planners and managers. We propose to assess the effects of an invasive species along an invasion gradient in a typical urban habitat. We focused on the Japanese knotweed (Reynoutria japonica Houtt.), a widespread invasive species in Europe and North America. We considered eight urban wastelands invaded by this species in the heart of the Greater Paris Area, France. On each site, we ran four transects from the centre of the Japanese knotweed patch towards the uninvaded peripheral vegetation. We recorded the flora using the line intercept method, and several soil parameters (thickness of A horizon, abundance of earthworm casts, topsoil Munsell value, pH) every metre along each transect. The A horizon was thicker and the topsoil darker under R. japonica canopy. Thus, this invasive plant species seemed to influence soil organic matter pool. However, our results also steadily showed that R. japonica locally excluded and/or severely reduced the cover of many plant species through competition. Our study clarified the local effects of R. japonica: an influence on the soil organic matter, and a severe negative impact on wasteland plant communities. We suggest implications in both conservation and restoration ecology.