Background: Diffuse large B cell lymphoma (DLBCL) is the most common histologic subtype of non-Hodgkin lymphoma. Secondary involvement of the heart is seen late in advanced cases and it is uncommon for DLBCL to present as a cardiac mass.Case Presentation: A 26-year-old female patient presented with progressive shortness of breath, facial swelling, and lower limb swelling. Echocardiography showed a large right atrial mass that was obstructing the tricuspid valve. Computed topography (CT) scan confirmed the findings and a large mediastinal mass with a filling defect in the right atrium occluding the distal Superior Vena Cava was identified. Histopathology showed DLBCL. She received 6 cycles R-CHOP chemotherapy (rituximab, cyclophosphamide, epirubicin, vincristine, and prednisone) with good response and complete resolution of the mass.Conclusions: DLBCL can present with atypical, uncommon sites as the first presentation, such as the heart. Early diagnosis and appropriate management is crucial given the poor outcome with late presentation. A high index of suspicion and the proper investigations including CT scan is recommended to allow for early intervention and favorable outcomes as what happened with the case under discussion.