The REEF Lionfish Sightings app was developed for the Reef Environmental Education Foundation (REEF), a marine conservation non-profit, leading the charge against invasive lionfish. Based on the Great Barrier Reef Marine Park Authority’s Wild About Whales app with support from the US Fish and Wildlife Service, the map-based Lionfish Sightings app aims to connect lionfish hunters with recent sightings and removal reports throughout the eastern US, Caribbean, and Gulf of Mexico. Lionfish are an invasive marine species causing severe impacts to native marine species and ecosystems. Removal efforts are showing promise in keeping local lionfish populations down and minimizing their impact. Data on lionfish sightings and removal efforts are kept active on the app for 30 days and then archived for research and management purposes. Additional lionfish information and links to resources are also provided.