Mesodermal cells signal to neighboring epithelial cells to modulate their proliferation in both normal and disease states. We adapted a Caenorhabditis elegans organogenesis model to enable a genome-wide mesodermal-specific RNAi screen and discovered 39 factors in mesodermal cells that suppress the proliferation of adjacent Ras pathway-sensitized epithelial cells. These candidates encode components of protein complexes and signaling pathways that converge on the control of chromatin dynamics, cytoplasmic polyadenylation, and translation. Stromal fibroblast-specific deletion of mouse orthologs of several candidates resulted in the hyper-proliferation of mammary gland epithelium. Furthermore, a 33-gene signature of human orthologs was selectively enriched in the tumor stroma of breast cancer patients, and depletion of these factors from normal human breast fibroblasts increased proliferation of co-cultured breast cancer cells. This cross-species approach identified unanticipated regulatory networks in mesodermal cells with growth-suppressive function, exposing the conserved and selective nature of mesodermal-epithelial communication in development and cancer. •Mesodermal suppressors of epithelial proliferation identified in C. elegans•Mesoderm-epithelial signaling components are highly conserved across species•Depletion of candidates in murine fibroblasts causes mammary hyper-proliferation•Human orthologs of identified genes are altered in breast cancer stroma Liu, Dowdle, Khurshid, Sullivan et al. identify mesodermal factors in C. elegans that suppress growth of adjacent epithelial cells. These candidates are shown to have conserved function in murine contexts and to be altered in human breast cancer stroma.