Urban greenspace is vital in fulfilling people's nature needs. Informal urban greenspace (IGS) such as vacant lots, street or railway verges and riverbanks is an often-overlooked part of the natural urban landscape. We lack a formal definition of IGS and a comprehensive review of knowledge about IGS and its role for urban residents. This paper advances a formal definition and typology of IGS that can be applied globally. Based on this definition, a total of 65 peer-reviewed papers in English (57), Japanese (7) and German (1) were reviewed. We analyzed this literature for its temporal trends, spatial patterns, studied IGS types, methods used and key authors, and summarized the individual research papers' findings concerning IGS. Results show IGS plays an important role for urban residents, but also highlight limitations and problems in realizing IGS' full potential. Research papers focused on perception, preferences, value and uses of IGS. Residents could distinguish between formal and informal greenspace. They preferred a medium level of human influence in IGS. The analysis of patterns in the literature reveals: a marked increase in publications in the last 20 years; a strong geographical bias towards the USA; and a lack of multi-type IGS studies including all IGS types. Publications outside of scholarly research papers also make valuable contributions to our understanding of IGS. Our results suggest IGS is emerging as an important sub-discipline of urban greening research.