Simple Summary: Invasive alien species are a major element of global change and represent a threat to global biodiversity and ecosystem integrity. Early detection of invasive species is of capital importance for management success. We describe the process by which the invasive whitefly, Aleurocanthus spiniferus (Hemiptera, Aleyrodidae), listed as a European quarantine organism, was detected in France. The first observation was made by a volunteer who reported a picture of an adult Aleurocanthus spiniferus on the Inventaire National du Patrimoine Naturel (INPN Espèces), a citizen science resource developed by l’Office Français de la Biodiversité and the French Muséum National d’Histoire Naturelle. We outline the sequence of actions that led to the official reporting of this species in France and show how incursion preparedness contributed to a rapid response. This case exemplifies how citizen science can contribute to the early detection of invasive species and highlights the importance of informing both the general public and professionals about major environmental issues. Abstract: We describe the process by which the quarantine whitefly, Aleurocanthus spiniferus (Hemiptera, Aleyrodidae), was detected in France. The initial observation was made by a volunteer who reported a picture of an adult in the Inventaire National du Patrimoine Naturel (INPN Espèces), a citizen science resource developed by l’Office Français de la Biodiversité and the French Muséum National d’Histoire Naturelle. The specimen was suspected to be A. spiniferus from this picture by one of the expert entomologists in charge of the Hemiptera group validation. Once the species was identified, it was mounted on a slide and the information was officially passed on to the ministry in charge of agriculture via a communication channel set up in advance for this type of situation. The ministry then triggered the regulatory actions planned in the event of the suspected detection of quarantine organisms. Sampling was quickly carried out and the specimens collected on this occasion were formally identified as belonging to the species A. spiniferus. This led to the formalization of an outbreak in France. This sequence of decisions took just two months from the first observation to the implementation of a management plan. This case presents how incursion preparedness contributes to a rapid response. Furthermore, this case exemplifies how citizen science can contribute to the early detection of invasive species and highlights the importance of informing both the general public and professionals about major environmental issues.