Objective To determine whether, beyond the conventional radiotherapy-based time anchor, the interval from the last immunotherapy administration to MRI reassessment also constitutes a key time anchor influencing assessment accuracy in patients with proficient mismatch repair/microsatellite-stable (pMMR/MSS) mid-to-low locally advanced rectal cancer treated with neoadjuvant chemoradiotherapy followed by immunotherapy (inCRT). Methods This retrospective study included 406 patients who underwent radical surgery after neoadjuvant therapy at Peking Union Medical College Hospital between June 2022 and April 2026. The inCRT cohort (n=136) was the primary analysis cohort, and the neoadjuvant chemoradiotherapy (nCRT) cohort (n=270) served as the reference. Pathologic suitability for organ preservation was defined as ypT0-1N0, and MRI suitability as MRI tumor regression grade (mrTRG) 1-2 with no suspicious residual lymph nodes (ymrN0). Concordance between MRI and pathology was considered an accurate assessment and served as the primary endpoint. The MRI reassessment interval was defined as the time from completion of the last relevant antitumor treatment to MRI reassessment; the interval from completion of radiotherapy to MRI was examined in sensitivity analyses. Multivariable Logistic regression and restricted cubic spline (RCS) models were used to evaluate time-effect associations of diagnostic performance. Results In the inCRT cohort, the median age was 60 years (Q1, 52; Q3, 67), 89 patients (65.4%) were male, 29 (21.3%) had baseline mrT4 disease, 83 (61.0%) had mrN2 disease, 63 (46.3%) had mesorectal fascia involvement, and 76 (55.9%) had extramural vascular invasion; 107 patients (78.7%) received long-course radiotherapy. 35 patients (25.7%) met the MRI criteria for suitability for organ preservation, whereas 66 patients (48.5%) met the pathologic criterion. The accuracy, sensitivity, and specificity of MRI assessment were 65.4%, 40.9%, and 88.6%, respectively. The interval from the last immunotherapy administration to MRI was longer in patients with accurate than inaccurate assessments [26 (17, 36) vs. 18 (9.5, 33) days, P=0.017]. Each 7-day increase in the interval was associated with greater odds of an accurate MRI assessment (adjusted OR, 1.273; 95%CI 1.025-1.600; P=0.029). RCS analysis showed a significant overall association between the interval and assessment accuracy (overall P=0.034), without evidence of nonlinearity (P for nonlinearity=0.158). Mean adjusted predicted accuracies during weeks 1-2, 3-4, and 5-6 after the last immunotherapy administration were 50.1%, 69.7%, and 72.6%, respectively. Accuracy during weeks 3-4 was 19.7 percentage points higher than during weeks 1-2 (95%CI 3.0-36.9 percentage points; P=0.046), whereas no further significant improvement was observed during weeks 5-6 (P=1.000). When the interval was anchored to completion of radiotherapy, no significant time-dependent associations were observed for accuracy, sensitivity, or specificity in the inCRT cohort. In the nCRT cohort, the interval was shorter in patients with accurate assessments [15.5 (5, 25) vs. 21 (10, 28.75) days, P=0.018], but neither Logistic regression nor the overall RCS test showed improved diagnostic performance with increasing interval. Conclusion In patients with pMMR/MSS locally advanced rectal cancer treated with neoadjuvant chemoradiotherapy followed by immunotherapy, an appropriate delay in MRI reassessment after the last immunotherapy administration may improve identification of patients pathologically suitable for organ preservation. Reassessment at 3-4 weeks may provide a reasonable balance between assessment reliability and timely subsequent decision-making and warrants further prospective validation.