Due to its uncommon nature, primary ovarian lymphoma has no clinical particularities and can be confused with other ovarian cancers. It poses a twofold diagnostic and therapeutic challenge. An anatomopathological and immunohistochemical study is the crucial step in the diagnosis. Our case was a 55-year-old woman diagnosed with an Ann Arbor stage II E ovarian non-Hodgkin's lymphoma who initially presented with a painful pelvic mass. This case reflects the major role of an immunohistochemical study in the diagnosis workup, leading to the appropriate management of such rare tumors.