Occasionally, heterotopic splenic tissue can occur in the renal fossa secondary to splenosis following splenic trauma or splenectomy. More rarely, it can represent a developmental anomaly secondary to the fusion of splenic and renal tissues. Splenorenal fusion can present as a renal mass, mimicking primary or secondary renal neoplasms on imaging studies, and patients can also present with symptoms of hypersplenism (anemia). We report a case of splenorenal fusion in a 51-year-old woman who initially presented with lithium toxicity, anemia, thrombocytosis, and a large renal mass that mimicked a primary renal neoplasm. The possible embryologic origin of splenorenal fusion, effects of lithium toxicity, and utility of various imaging modalities are discussed. The literature on renal heterotopic splenic tissue is also briefly reviewed.