Background: Standard neoadjuvant long course chemoradiotherapy (LCCRT) for locally advanced rectal cancer (LARC) results in a complete pathological response rate of 10-30%: but 20-40% of patients (pts) are non-responders, 10-15% have local recurrence. Tumoural immune infiltrates are predictive of response. Preclinical studies show that radiotherapy (RT) via interferon signaling is immuno-stimulatory, enhancing local/distant tumour cell death. RT also stimulates PDL1 production and the immunosuppressive activity of myeloid derived suppressor cells. Hence PDL1 inhibition may be required to enhance the immuno-stimulatory effects of RT. Hypothesis: In pts with resectable LARC, the anti-PDL1 antibody Avelumab post LCCRT may enhance the pathological/imaging response rates whilst potentially reducing local/distant relapse rates. Methods: (1) Trial Design: Phase II single arm trial, across 6 Australian sites (2) Endpoints: (a) Primary; Pathological response rate post-LCCRT, as documented by central pathologist, (b) Secondary; MRI/FDG PET imaging responses at 8 weeks post LCCRT (pre-surgery). Toxicity. (c) Exploratory; Tumoural immune cell subsets/checkpoint expression (by multiplex immunohistochemistry and in-vitro functional assays) and ctDNA analysis at baseline and during treatment. Distant relapse-free survival and the documentation of sites of relapse. (3) Sample size: An increase in the proportion of pathological complete responses by > 25% (from 10% to 35%) will be considered clinically important. Power = 90%, α = 0.05, 41 pts are required– an additional 4 pts to allow for drop-out. Total sample size = 45pts. Treatment: All pts to receive standard LCCRT (50.4Gy RT plus 5FU [225mg/m²/day/CI] or Capecitabine [825mg/m² BID on RT days] over 5.5 weeks). Post LCCRT (prior to surgery), pts receive 4 cycles Avelumab (10mg/kg, q2 weeks). Surgical resection 10-12 weeks post LCCRT. Fresh tumour biopsy and ctDNA sampling pre LCCRT, pre Cycle 1 Avelumab and at surgery. Response by FDG PET and pelvic MRI pre surgery. Pts to be followed up for 2 years. Major Inclusion Criteria: Pts with LARC, MRI stage T3b-4/N1-2/M0, planned for LCCRT followed by curative resection, tumoural lower border within 12cm from the anal verge, measurable disease (RECIST1.1), ECOG 0-1, adequate organ function and no contraindications to Avelumab therapy. Current Enrolment: 11 of the planned 45 patients enrolled. Clinical trial information: NCT03299660