Background: Adenocarcinomas arising from the distal one third of the transverse colon, splenic flexure, descending colon, sigmoid colon and rectum are often grouped together due to their hindgut embryologic origin and referred to as left-sided colorectal cancer (CRC). Rectal cancer represents a subset of CRC that has distinct differences in anatomical location, clinical behavior, prognosis and molecular background. In patients with left sided colon cancer (LSCC), the expression of exosomal marker CD63 was reported to be higher in the adjacent normal mucosa (ANM) compared to the tumor (224 vs 154, p = 0.0001). Here, we explored the pattern of CD63 expression using immunohistochemistry in patients with rectal cancer in comparison with patients with LSCC. Methods: Between 2015 and 2018, 53 patients underwent rectal cancer biopsy/resection and had available tissues for CD63 IHC staining. Two pathologists independently scored CD63 expression in the tumor and ANM. Staining intensity was graded from 1-3. Staining percentage was estimated in 10% increments. Mean quick-score (Q-score) was calculated (intensity x percentage). Paired t test was used for statistical analysis. Results: Median age was 60 (range 34-80). Females represented 26%. Caucasians and African Americans represented 74% and 26%, respectively. In patients with rectal cancer, the mean CD63 expression was higher in ANM compared to their expression in the tumor (147 vs 113, p = 0.0012). Compared to patients with LSCC (N = 30), the mean CD63 expression in patients with rectal cancer was lower in the ANM (224 vs 147, p < 0.0001) and in the tumor (154 vs 113, p = 0.01). Conclusions: In our cohort of patients with rectal cancer, exosomal marker CD63 expression was lower in tumor compared to ANM. This observation was similar to our previously reported findings in patients with LSCC. Compared to patients with LSCC, patients with rectal cancer had lower expression of CD63 in the tumor and ANM. To our knowledge, this is the first study to explore exosomal marker CD63 expression using IHC in patients with rectal cancer.