Calnuc is an ubiquitous Ca(++)-binding protein found in the cytoplasm where it binds different Galpha subunits, in the Golgi lumen where it constitutes a major Ca(++) storage pool, and outside the cell. We identified LDLR-related protein 9 (LRP9) as the first transmembrane protein shown to interact directly with Calnuc. LRP9 is a member of a new subfamily of the LDLR superfamily that cycles between the trans-Golgi network (TGN) and endosomes through a mechanism dependent on clathrin adaptor GGA proteins. The aim of the present study was to characterize the interaction between Calnuc and LRP9. Various biochemical assays showed that the N-terminus of Calnuc interacts with an arginine-rich region in the cytosolic tail of LRP9. Confocal microscopy showed that Calnuc colocalizes with LRP9 at the surface of the TGN and early endosomes. Depletion of Calnuc by small interfering RNA (siRNA) missorted LRP9 in the late endosome/lysosome compartments and enhanced its lysosomal degradation. This phenotype was rescued by the expression of siRNA-resistant wild-type Calnuc as well as cytoplasmic Calnuc, indicating that the cytoplasmic pool of Calnuc is involved in LRP9 endosomal sorting to prevent the delivery of LRP9 to lysosomes. This is the first report showing that Calnuc plays a role in receptor trafficking.