Research by Project Seagrass, formed by scientists from Cardiff University and Swansea University, has shown the meadows are in a "perilous state".
Seagrasses are plants that form dense underwater beds in shallow water.
It is hoped people will use the app to help scientists with monitoring, conservation and education efforts.
"The app provides ocean enthusiasts around the world with an opportunity to become citizen scientists who contribute to marine conservation with just a few taps of their phone", said Benjamin Jones, project co-founder and research assistant at Cardiff University's Sustainable Places Research Institute.
The team hopes to create a more comprehensive picture of seagrass meadows around the globe.
It hopes it will inspire new scientific research and conservation measures that can help protect ocean habitats.
Marine conservationists have launched an app to encourage the public to identify and monitor underwater seagrass meadows.