Inclusion Criteria:  Histologically proven adenocarcinoma of the colon or rectum which is MMR-d by IHC or MSI-H by PCR (or microsatellite testing if routine practice).  Patient is fit (ECOG 0-1) and eligible for planned curative surgery in keeping with NICE guidelines and considered fit/suitable for adjuvant chemotherapy as per local site investigator's discretion based on:  Radiological node positive T1-4 CRC or high risk T3 defined as EITHER ≥ 5mm of extramural depth of invasion OR unequivocal EMVI on imaging (regardless of depth) or T4 disease Patients with rectal cancer are eligible if it is determined that neoadjuvant chemo-radiotherapy is not required to achieve a R0 resection. Patients presenting with acute colonic obstruction may enter the trial only after obstruction is relieved by a successful defunctioning stoma/stent, and when recovered to a fitness level consistent with the other eligibility criteria  Adequate bone marrow function:  White Blood Cell >3.0 x 10^9/L; Absolute neutrophil count ≥1.5 x 10^9/L Platelets ≥100 x 10^9/L. Haemoglobin ≥90 g/L Note Anaemia (Hb <100 g/L) is not an exclusion, but should be corrected by transfusion prior to surgery and chemotherapy.  Adequate renal function:  GFR >50 mL/min estimated using validated creatinine clearance calculation (e.g. Cockroft-Gault) NB If the calculated creatinine clearance is < 50 mL/min, a formal 24 hour urine collection or isotope clearance must be carried out demonstrating GFR ≥ 50 mL/min as per institutional standards  Adequate liver function:  Total bilirubin < 1.5 times Upper Limit of Normal (ULN) OR direct bilirubin ≤ULN for participants with total bilirubin levels >1.5 × ULN  AST and ALT ≤ 2.5 × ULN  Adequate coagulation:  International normalized ratio (INR) OR prothrombin time (PT) and Activated partial thromboplastin time (aPTT) ≤1.5 × ULN unless participant is receiving anticoagulant therapy as long as PT or aPTT is within therapeutic range of intended use of anticoagulants  Aged ≥18 years Able and willing to provide written informed consent Willing to use highly effective contraception for the duration of trial treatment and for 120 days after last dose of pembrolizumab  Exclusion Criteria:  Any patient for whom radiotherapy is advised by the MDT Strong evidence of distant metastases or peritoneal nodules (M1) Prior therapy with an anti-PD-1, anti-PD-L1 or anti-PD-L2 agent, or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (e.g. CTLA-4, OX-40, CD137)  Prior systemic anti-cancer therapy including investigational agents within 4 weeks prior to registration.  (NB: Participants must have recovered from all AEs due to previous therapies to ≤Grade 1 or baseline, with the exception of alopecia. Participants with ≤Grade 2 neuropathy may be eligible.) (NB: If participant received major surgery, they must have recovered adequately from the toxicity and/or complications from the intervention prior to starting study treatment.)  Has received a live vaccine or live-attenuated vaccine within 30 days prior to registration (seasonal flu vaccines that do not contain live virus are permitted). Administration of killed vaccines is allowed Any investigational agents or investigational devices within 4 weeks prior to registration Note: Participants who have entered the follow-up phase of an investigational study may participate as long as it has been 4 weeks after the last dose of the previous investigational agent. Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (dosing exceeding 10mg daily of prednisolone or equivalent), or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment Note: the use of physiologic doses of corticosteroids may be approved after consultation with UCL CTC. Patients with concurrent or previous malignancy that could compromise assessment of the primary or secondary endpoints of the trial Has known active CNS metastases and/or carcinomatous meningitis. Has severe hypersensitivity (≥Grade 3) to pembrolizumab and/or to any of its excipients. Has previous severe or life-threatening skin adverse reaction with other immune-stimulatory anticancer agents  Has active autoimmune disease that has required systemic treatment in the past 2 years (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive drugs).  NB: Replacement therapy (e.g. levothyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, etc.) is permitted.  History of (non-infectious) pneumonitis/interstitial lung disease that required steroids, or current pneumonitis/interstitial lung disease Active infection requiring systemic therapy Known history of Human Immunodeficiency Virus (HIV). NB: Testing for HIV for the NEOPRISM-CRC trial is not mandatory, however if this test has been done the result should be known prior to registration. Known history of or is positive for hepatitis B (hepatitis B surface antigen [HBsAg] reactive) or known active hepatitis C (defined as hepatitis C virus [HCV] RNA [qualitative] is detected) Note: Without known history, testing is required to determine eligibility. Hepatitis C antibody testing is allowed for screening purposes in sites where HCV RNA is not part of standard of care. Known history of active TB (Mycobacterium tuberculosis). Has had an allogenic tissue/solid organ transplant. Has peritonitis (secondary to perforated tumour) Has a colonic obstruction that has not been defunctioned or stented History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the subject's participation for the full duration of the study, or is not in the best interest of the subject to participate, in the opinion of the treating investigator. Known psychiatric or substance abuse disorder that would interfere with the participant's ability to cooperate with the requirements of the study. Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the pre-screening or screening visit through 120 days after the last dose of pembrolizumab