Background: A growing body of evidence suggests that conventional chemotherapy may not be effective in mismatch repair deficiency (dMMR)/microsatellite instability-high (MSI-H) locally advanced rectal cancer (LARC). Alternative strategies, such as immunotherapy, are currently being investigated both in the neoadjuvant and adjuvant setting. Furthermore, immunotherapy is an attractive alternative to the use of combination chemotherapy regimens when treating synchronous primary cancers such as in the setting of inherited cancer syndromes.Case Description: Here we present a case of a middle-aged woman diagnosed with dMMR/MSI-H locally advanced rectal cancer with synchronous upper tract urothelial cancer secondary to Lynch syndrome.The patient was first treated using neoadjuvant chemotherapy followed by chemoradiation, resulting in only a partial pathologic response. Following surgery, the patient was treated with adjuvant combination immunotherapy with nivolumab, a PD-1 inhibitor, and ipilimumab, a CTLA-4 inhibitor, resulting in a durable disease-free interval of nearly 21 months.Conclusions: This case report illustrates the importance of determining dMMR/MSI-H status in LARC and the consideration of immunotherapy (particularly with synchronous primaries as seen in inherited cancer syndromes), reviews the current literature, and calls for further investigation into the use of neoadjuvant and adjuvant immunotherapy in locally advanced rectal cancer along with upper tract urothelial carcinoma (UTUC).