Sustainable tourism minimises the adverse impact of tourism on the natural environment and local culture while stimulating the socioeconomic development of regions or even countries. Geoportals and (geo)informational mashup portals significantly affect sustainable tourism planning through modern computer solutions for more sustainable planning of tourist activities on the demand and supply sides. This study had two research aims: (1) to develop a checklist for assessing the functionality of tourist geoportals and evaluate it and (2) to assess the inventory of functions useful for sustainable tourism planning available at selected geoportals with the checklist. The aims were pursued with an original research tool, the Functionality Assessment Checklist (FAC). The FAC is a set of original criteria useful for assessing the quality of tourist geoportals. This study investigated the following research questions: (Q1) What functions should be included on a checklist for assessing the functionality of geoportals useful for sustainable tourism planning? (Q2) What functions should be included in a geoportal to facilitate sustainable tourism planning? The original contributions of this article are (1) the checklist for assessing the functionality of tourist geoportals and (2) the assessment of the impact of geoportal’s functionality on the possibility of planning sustainable tourism. The functionality assessment revealed that the tested geoportals have most of the tourism, informational, educational, and use-related functions. This suggests they can be useful for sustainable tourism planning.