An urban tree monitoring programme is a management procedure that determines the performance status of trees by conducting an inventory of the number of trees, their condition, their structure, and other quantitative or qualitative characteristics. The ability to successfully carry out a monitoring programme is highly dependent on precise data. At the same time, comprehensive and strategic urban tree policies require accurate baseline and trending data. Accordingly, there is a need for an improved assessment technique for tree monitoring. Due to the increasing number of tree vandalism incidents in urban areas, the immediate objective of this research was to determine the criteria for a precise tree vandalism assessment technique. A modified Delphi method was adopted to obtain the most reliable consensus among tree care experts through a series of questionnaires. Prior to conducting the Delphi survey, the initial tree vandalism criteria were identified through a preliminary survey conducted by means of the photoelicitation technique. Experts provided the input for generating the criteria, and at the end of the second round of the Delphi survey, the experts reached a consensus on a set of criteria, which included 10 criteria related to aspects of specific motives and actions, 11 criteria related to aspects of ideologies and practices, and 11 other criteria related to the aspect of victim of circumstances. The selected criteria can be applied in a monitoring programme to assess the incidence of tree vandalism.