Marginal zone B cell lymphomas (MZL) are biologically heterogeneous, rarely demonstrating biclonality, complex cytogenetic abnormalities, or T cell predominance. We report a case of biclonal splenic MZL, T cell-rich variant with an abnormal karyotype that progressed to large B cell lymphoma. A 74-year-old female presented with pancytopenia, weight loss, fever, and splenomegaly. Microscopically, the spleen revealed an extensive, vaguely nodular lymphoid proliferation, composed of small lymphocytes, majority of which were reactive T cells. B cells were mostly small and < 5% of total lymphocytes. Focal follicular dendritic cell networks were present, but germinal centers were absent. Flow cytometric analysis revealed two distinct CD5 and CD10 negative B cell clones, one kappa positive and one lambda positive. Conventional cytogenetic analysis revealed the following abnormal karyotype: 47,XX,add(7)(q36),del(7)(q22),add(21)(p11.2),+mar[10]/46,XX[10]. Immunoglobulin heavy chain gene rearrangement showed two monoclonal peaks of different magnitude, consistent with biclonality. Overall, these features favored a low-grade splenic MZL. The staging bone marrow biopsy was normocellular with few interstitial lymphoid aggregates, composed of small lymphocytes, consistent with minimal involvement by low-grade B cell lymphoma. The patient improved without adjuvant chemotherapy; however, 12 months later, she developed anemia and lymphocytosis. Subsequent bone marrow biopsy showed extensive involvement by a large B cell lymphoma and complex karyotype with the previously identified abnormalities, as well as additional numerical and structural aberrations, consistent with cytogenetic evolution and biclonal gene rearrangement. These findings were consistent with transformation. This case demonstrates a unique pathological presentation of splenic MZL with disease progression, highlighting the importance of an integrated approach for lymphoma classification and the difficulties in diagnosing such cases.