This book constitutes the refereed proceedings of the Third International Conference on GeoSpatial Semantics, GeoS 2009, held in Mexico City, Mexico in December 2009. The 10 revised full papers presented together with 2 keynote speeches were carefully reviewed and selected from 19 submissions. The papers are organized in topical sections foundations on geo-semantics; formal representation of geospatial data; semantics-based information retrieval and recommmender systems; integration of sematics into spatial query processing; and geo-ontologies and applications.