Thousands of recently discovered microproteins represent a new frontier in the search for functional and disease-causing genes. Though shorter than canonical proteins, some microproteins contain signal peptides and are predicted to produce secreted peptides. However, whether any of the microprotein-derived secreted peptides possess biological activity remains underexplored. Here, we screen a small library of secreted peptides from the microproteome by measuring signaling downstream from GPCRs. This approach identified several cAMP-stimulating peptides, including a secreted peptide from a "non-coding" HLA complex P5 RNA (HCP5). The HCP5-secreted peptide (HCP5-SP) is encoded by a small open reading frame embedded in the HCP5 mRNA. In vitro assays with synthetic HCP5-SP and HCP5-SP analogs validated its cAMP-stimulating activity and revealed the necessity for the wild-type C-terminal sequence for activity. Furthermore, HCP5-SP promotes the proliferation of HEK293T cells, providing an alternative mechanism that might explain some of the cancer biology associated with HCP5 mRNA. In summary, this work establishes a workflow for the preliminary identification of bioactive microproteins and demonstrates that the vast, largely untapped microproteome is a source of novel bioactive endogenous peptides.