Inclusion Criteria:  Histologically confirmed diagnosis of adenocarcinoma of the rectum Age ≥ 18 years ECOG performance status 0-1 cT3N0 or cT1-3N1 5cm from the anal verge Rectal cancer amenable to total mesorectal excision No evidence of distant metastases No prior pelvic radiation therapy No prior chemotherapy or surgery for rectal cancer No infections requiring systemic antibiotic treatment Hgb >8.0 gm/dL, PLT > 150,000/mm3, total bilirubin ≤ 1.5x upper limit of normal, AST ≤ upper limit of normal, ALT ≤ 3x upper limit of normal Female participants or reproductive potential, defined as not surgically sterilized and between menarche and 1 year post menopause, must have a negative serum pregnancy test within 4 weeks prior to initiation of study treatment Female participants of reproductive potential and male participants with female partners of reproductive potential must remain abstinent (refrain from heterosexual intercourse) or use highly effective contraceptive measures from the start of study treatment until 30 days after the last dose of etrumadenant, 90 days after the last dose of zimberelimab, whichever is longer Women with childbearing potential who are negative for pregnancy (urine or blood) and who agree to use effective contraceptive methods. A woman of childbearing potential is defined by one who is biologically capable of becoming pregnant. Reliable contraception should be used from trial screening and must be continued throughout the study. Male subjects must also agree to use effective contraception.  Exclusion Criteria:  Recurrent rectal cancer Primary unresectable rectal cancer is defined as a primary rectal tumor which, on the basis of either physical exam or pelvic MRI, is demed to be adherent or fixed to adjacent pelvic structures (en bloc resection wll not be achieved with negative margins). ≥4 regional lymph nodes each ≥10 mm on pelvic MRI Suspected T4 tumor Involved radial margin Serum creatinine level >1.5x the upper limit of normal Patients who have received prior pelvic radiotherapy QTc ≥480 msec using Fredericia's QT correction formula  Due to the potential risk for drug-drug interactions with etrumadenant, participants must not have had:  Treatment with known BCRP substrates with a narrow therapeutic window, administered orally (e.g., prazosin, rosuvastatin) within 4 weeks or 5 half-lives of the drug (whichever is shorter) prior to initiation of and throughout study treatment Treatment with known P-gp substrates with a narrow therapeutic window, administered orally (e.g., digoxin) within 4 weeks or 5 half-lives of the drug (whichever is shorter) prior to initiation of study treatment Treatment with known strong CYP3A4 inducers (e.g., rifampin, phenytoin, carbamazepine, phenobarbital, and St. John's Wort) and strong CYP3A4 inhibitors (e.g., clarithromycin, grapefruit juice, itraconazole, ketoconazole, posaconazole, telithromycin, and voriconazole) within 4 weeks or 5 half-lives of the drug (whichever is shorter) prior to initiation of study treatment Any gastrointestinal condition that would preclude the use of oral medications (e.g., difficulty swallowing, nausea, vomiting, or malabsorption) Prior treatment with an agent targeting the adenosine pathway History of severe allergic reactions to chimeric or humanized antibodies or fusion proteins Patients with a history of any arterial thrombitic event within the past 6 months, - Patients with any other concurrent medical or psychiatric condition or disease which, in the investigator's judgment would make them inappropriate candidates for entry into this study Patients with a history of prior malignancy within the past 5 years, except for adequately treated basal cell or squamous cell skin cancer, or in situ cervical cancer. Patients with a history of thrombotic episodes, such as deep venous thrombosis, pulmonary embolus, MI or CVA occurring more than 6 months prior to enrollment may be considered for protocol participation, provided they are on stable doses of anticoagulant therapy. Patients who are anticoagulated for atrial fibrillation or other conditions may participate, provided they are on stable doses of anticoagulant therapy. Patients receiving other anticancer or experimental therapy. No other experimental therapies (including chemotherapy, radiation, hormonal treatment, antibodiy therapy, immunotherapy, gene therapy, vaccine therapy, angiogenesis inhibitors, matrix metalloprotease inhibitors, thalidomide, anti-VEGF/Flk-1 monoclonal antibody, or other experimental drugs) of any kind are permitted while the patient is receiving study treatment. Women who are pregnant or breastfeeding. Women of childbearing potential who are unwilling or unable to use an acceptable method of birth control to avoid pregnancy for the entire study period and for up to four weeks after the study.