An occulter is an instrument designed to suppress starlight by diffraction from its edges; most are designed to be circular, with a set of identical "petals" running around the outside. Proposed space-based occulters are lightweight, deployed screens tens of meters in diameter with challenging accuracy requirements. In this paper we describe the design of an occulter for the THEIA mission concept. THEIA consists of a 4-meter telescope diffraction limited to 300 nm, and a 40-meter external occulter to provide high-contrast imaging. Operating from 250 to 1000 nm, it will provide a rich family of science projects, including exoplanet characterization, ultraviolet spectroscopy, and very wide-field imaging. Originally conceived of as a hybrid system employing both an occulter and internal coronagraph, THEIA now uses a single occulter to achieve all of the starlight suppression but at two different distances from the telescope in order to minimize size and distance. We describe the basic design principles of the THEIA occulter, its final configuration, performance, and sensitivity.