Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for distributed nearest-neighbor search for image retrieval are disclosed. An example distributed image retrieval system includes a root server and a plurality of leaf servers. The root server controls the clustering of images in the image corpus, and determines the assignment of image clusters to a collection of leaf servers. Each leaf server is independently configured to perform a two-stage nearest-neighbor search in a locality-sensitive hash space for a received query image within the leaf server's assigned images clusters. Thus, the distributed image retrieval system is modular and amendable to tuning, modifications, duplication, and replacement on a per-server basis. In addition, the design of the image retrieval system allows flexible load balancing among the leaf servers.