Human endogenous retroviruses (HERVs) are remnants of ancient infections that comprise ~8% of the human genome. The HERV-K envelope glycoprotein (Env) is aberrantly expressed in cancers, autoimmune disorders, and neurodegenerative diseases, and is targeted by patients own antibodies. However, a lack of structural information has limited molecular and immunological studies of the roles of HERVs in disease. Here, we present cryo electron microscopy structures of stabilized HERV-K Env in the prefusion conformation, revealing a distinct fold and architecture compared to HIV and simian immunodeficiency virus. We also generated and characterized a panel of monoclonal antibodies with subunit and conformational specificity, serving as valuable research tools. These antibodies enabled structure determination of the postfusion conformation of HERV-K Env, including its unique tether helix, and antibody-bound prefusion Env. Together, these results provide a structural framework that opens the door to mechanistic studies of HERV-K Env and tools for its evaluation as a potential therapeutic target.Structural snapshots and antibodies illuminate a viral protein within the dark matter of the genome that may be active in disease.