Non-native invasive species frequently appear in urban and non-urban ecosystems and may become a threat to biodiversity. Some of these newcomers are introduced accidentally, and others are introduced through a sequence of events caused by conscious human decisions. Involving the general public in biodiversity preservation activities could prevent the negative consequences of these actions. Accurate and reliable data collecting is the first step in invasive species management, and citizen science can be a useful tool to collect data and engage the public in science. We present a case study of biological recording of tree of heaven (Ailanthus altissima (Mill.) Swingle) using a participatory citizen model. The first goal in this case study was to develop a cheap, widely accessible, and effective inventory method, and to test it by mapping tree of heaven in Croatia. A total of 90.61 km of roads and trails was mapped; 20 single plants and 19 multi-plant clusters (mapped as polygons) were detected. The total infested area was 2610 m2. The second goal was to educate citizens and raise awareness of this invasive species. The developed tool and suggested approach aided in improving invasive risk management in accordance with citizen science principles and can be applied to other species or areas.