Streets comprise a major part of urban land use, but the significance of street habitats for urban biodiversity remains poorly understood. In this study, urban street vegetation was investigated in Oulu, Finland, and Yokohama, Japan, to explore how plant species richness and species composition change among various street habitat types. All plant species were recorded separately for different street habitats along a maximum of 500 m of street sections. The species richness and the contributions of native and exotic species were evaluated based on species accumulation curves. Plant species composition was examined with Venn diagrams. A total of 145 and 273 species were recorded in Oulu and Yokohama, respectively. In Oulu, native species contributed more to the total diversity across habitat types, whereas in Yokohama, native and exotic species had similar contribution to the total diversity. There was a large overlap in species occurrence among habitat types, but the habitats also included many unique species. Thus, streetscape vegetation is a mixture of common urban species and some unique species, which are often overlooked as a part of the spontaneous plant diversity across streetscapes. We propose that streets including various habitat types may thereby create biodiverse urban streetscapes.