Human endogenous retroviruses (hERVs) are molecular remnants of ancient retroviruses that integrated into the human germline and now make up 8% of the human genome. The human ERVK3-1 locus was recently annotated as encoding a 109-amino acid microprotein homologous to a retroviral Rec domain. ERVK3-1 is ubiquitously expressed in normal human cells and tissues. However, because this locus was thought to be non-coding until recently, it is currently unknown whether the ERVK3-1 microprotein has a function in human cells. We demonstrate that the ERVK3-1 microprotein interacts with PPHLN1, a component of the HUSH complex, and contributes to target gene repression. We thus suggest that the ERVK3-1 Rec protein may have been co-opted by human cells for molecular sensing and/or feedback regulation of hERV expression.