To study the relation between changes in local plant communities and shifts in temperature, the temperature niches of the species need to be defined. We calculated the Species Temperature Index (STI) as a proxy for this niche. STI is defined as the mean annual temperature within the range of a species. In this paper, a method is described to calculate the STI for the European flora from open data provided by the Global Biodiversity Information Facility (GBIF) and WorldClim global climate data, for 7254 taxa of European vascular plants, bryophytes, algae, and ascomycetes including lichens. The algorithm accounts for incomplete and unbalanced species distribution data. The Community Temperature Index (CTI) is defined as the weighted mean of the STIs of a species assemblage. A 1 × 1 km CTI grid map for the Netherlands is presented as an example of the use of STIs.