The Mosquito Alert dataset includes occurrence records of adult mosquitoes. The records were collected through Mosquito Alert, a citizen science system for investigating and managing disease-carrying mosquitoes. Each record presented in the database is linked to a photograph submitted by a citizen scientist and validated by entomological experts to determine if it provides evidence of the presence of any of five targeted mosquito vectors of top concern in Europe (i.e. Aedes albopictus, Aedes aegypti, Aedes japonicus, Aedes koreicus, Culex pipiens). The temporal coverage of the database is from 2014 through 2022 and the spatial coverage is worldwide. Most of the records from 2014 to 2020 are from Spain, reflecting the fact that the project was funded by Spanish national and regional funding agencies. Since autumn 2020 the data has expanded to include substantial records from other countries in Europe, particularly the Netherlands, Italy, and Hungary, thanks to a human volunteering network of entomologists coordinated by the AIM-COST Action and to technological developments through the VEO project to increase scalability. Among many possible applications, Mosquito Alert dataset facilitates the development of citizen-based early warning systems for mosquito-borne disease risk. This dataset can be further re-used for modelling vector exposure risk or training machine-learning detection and classification routines on the linked images, to help experts in data validation and build up automated alert systems.