|Abstract||We propose a novel Cloud-based geolocation data service system, termed ‘Geopot’, for location-based mobile applications. The exponentially growing number of users of location-based mobile applications demand a data service that can easily be deployed and is scalable against a large volume of accesses from mobile devices across the world. The purpose of our work is to construct a scalable spatial data service that leverages the powerful benefits of a Cloud-based storage. We focus on highly scalable check-in and nearby search service, which is a common focus of location-based mobile applications. Our system comprises two parts: a local data service for indexing and storing geolocation data of check-ins to achieve a compact spatial index database. This is based on an in-memory R-tree and a local hash table, and it utilizes a Cloud storage that enables global accesses. The local data service maintains a compact spatial index, with tempo-spatial clustering, in which check-ins are grouped by their distance through a time window. A centroid of a cluster is used as a spatial index of R-tree, and members of the cluster are stored in the local networked hash table temporarily. This insures that R-tree remains in a compact size that can fit into the system memory. The data stored in the local hash table will be published into the Cloud storage to allow users to access it remotely with the same quality of service. Publishing the local data to a Cloud not only insures staying within the specified storage size limits of the local data service, but also promotes scalable access to the Cloud from mobile clients. Our contribution lies in the design of a new scale-out data service architecture and in implementing this for mobile applications. We encourage the building of a mobile application with our proposed system as well as a low-cost Cloud data service for linking to a large-scale spatial database.