Background: Pembrolizumab, an immune checkpoint inhibitor targeting programmed death 1 (PD-1), is widely employed in the treatment of various gastrointestinal (GI) cancers. This systematic review aims to assess the spectrum and incidence of immune-related adverse events (irAEs) associated with pembrolizumab monotherapy in GI cancer patients. Methods: A comprehensive search of PubMed/MEDLINE was conducted to identify full-text articles of clinical trials investigating pembrolizumab monotherapy in GI cancer patients. Two independent authors reviewed the articles and extracted data, with discrepancies resolved by consensus. Primary endpoints included the incidence of grade 3 or higher irAEs and the treatment discontinuation rate due to irAEs. Secondary endpoints encompassed the incidence of any-grade irAEs and specific irAEs such as pneumonitis, colitis, hepatitis, myositis, myocarditis, nephritis, pancreatitis, peripheral neuropathy, skin toxicity, endocrine toxicity, infusion reactions, and deaths. Results: After excluding duplicates and articles not meeting inclusion criteria, data extraction and analysis were performed on 25 selected articles. Common reasons for exclusion included concomitant use of chemotherapy or targeted therapy and trials involving non-GI cancer patients. The analysis included 2866 patients with a median age of 62 years, of which 29% were female. Tumor types included colorectal (13%), esophagogastric (46%), hepatocellular carcinoma (25%), and others (16%). The aggregate incidence of grade 3 or higher irAEs was 8.7%, with reported rates ranging from 3% to 41% across different trials. The incidence of any grade irAEs varied from 39% to 98%. The treatment discontinuation rate due to irAEs was 6.8%. The most common grade 3 or higher irAEs were hepatitis (3.6%), pneumonitis (0.8%), and colitis (0.7%). irAE-related deaths were infrequent (0.9%). Detailed irAE data are summarized in the Table. Conclusions: In GI cancer patients receiving pembrolizumab monotherapy, severe irAEs are rare. Treatment discontinuation and deaths attributed to pembrolizumab-related irAEs are infrequent, underscoring the safety profile of this therapy in patients with GI cancer.