This article explores a new model of urban practice that is not limited by environmental moralism by investigating theories of plastic, which is today’s prevailing environmental problem and also a ubiquitous strata of everyday life. In doing so, it investigates the following two things. The first is to speculate about the ontological dimensions of plastic. Often thrown away after a single use and thrown into the ocean in the form of microplastics, plastic is rubbish and also a toxic material influencing climate change on a global level. It is both a problematic material and an object to overcome. However, such an objectification is a result that does not pay enough attention to its ubiquity. By considering plastic as a crucial form of life, this article pays close attention to its ontological dimensions. The second thing is to examine “assemblage urbanism” as a new form of urban model that could be further explored in relation to the plastic ontology. Assemblage urbanism brings Gilles Deleuze’s notion of assemblage into urban discourses, thereby trying to rethink the conventional theories of the city, represented by its counterpart known as “critical urbanism” that highlights an analytical approach. This article examines the debate between those two camps, which appeared in serial issues of the journal City in 2011, as well as paying attention to the weak points of assemblage urbanism. What this article derives through the investigation is a theory called “plastic urbanism”, a new form of urban practice that is not subjugated by the Anthropocene as a grand narrative, but offers a microscopic and diagrammatic model of the city.