BACKGROUND: Inflammatory pseudotumor of the spleen and lepidic adenocarcinoma of the lung are uncommon slow-growing malignancies that have not been previously reported to occur concurrently. CASE PRESENTATION: We present the case of a 63-year-old Caucasian man who presented with a splenic inflammatory pseudotumor-like dendritic cell sarcoma and was found to have a concomitant invasive lepidic adenocarcinoma of the lung. The patient underwent laparoscopic splenectomy to address the splenic mass. Three months later, he underwent video-assisted thoracoscopic surgery, wedge resection, superior segmentectomy, and mediastinal lymph node dissection to manage the lung lesion. Final pathology revealed pT1c, N0, M0, stage IA3 lepidic adenocarcinoma. The patient received his post-splenectomy vaccinations and will repeat a computed tomography of the chest 6 months postoperatively for pulmonary surveillance. CONCLUSIONS: This report highlights the indication for surgical intervention in the management of splenic masses, as well as the importance of early operations for low-grade splenic lesions. The simultaneous occurrence of inflammatory pseudotumor of the spleen and lepidic adenocarcinoma of the lung sheds light on the need for comprehensive evaluation and multidisciplinary treatment strategies for patients with rare concurrent malignancies. This case report may also be corroborated by future similar reports that may unfold a discovery of a genetic association or syndromic disorder. This case underscores the critical role of surgical intervention and thorough evaluation in patients with rare concurrent malignancies, such as splenic inflammatory pseudotumor and lepidic adenocarcinoma. Future cases may reveal potential genetic or syndromic links, further guiding treatment and surveillance strategies.