Background/Objectives: To evaluate the concordance of dual-layer spectral CT (DLSCT) with MRI in locoregional staging of rectal cancer, focusing on 40 keV virtual monoenergetic reconstructions. Methods: This retrospective single-center study included 31 patients (mean age, 69 ± 13.3 years) with rectal cancer who underwent both MRI and DLSCT within 30 days before treatment. Venous-phase CT images were reconstructed at 40 keV. Assessed parameters included tumor location, longitudinal extent, distance from the anorectal junction and anal verge, mesorectal infiltration, circumferential resection margin (CRM) involvement, suspicious lymph nodes, extramural vascular invasion (EMVI), desmoplastic reaction, peritoneal involvement, and anal canal invasion. Two radiologists performed consensus readings. Paired continuous measurements were compared with the Wilcoxon signed-rank test. Agreement for categorical variables was evaluated using Cohen’s kappa; sensitivity, specificity, and accuracy were calculated using MRI as the imaging reference standard. Results: No significant differences were found between MRI and DLSCT for the continuous measurements evaluated. Complete agreement was observed for CRM involvement, anal canal invasion, peritoneal involvement, desmoplastic reaction, and suspicious lymph-node classification (accuracy, 100%; κ = 1.00). Agreement was almost perfect for mesorectal infiltration (κ = 0.84). For EMVI, DLSCT reproduced all MRI-positive classifications but generated two additional positive findings, resulting in 100% sensitivity, 71.4% specificity, 93.5% accuracy, and substantial agreement (κ = 0.79). Conclusions: DLSCT showed high concordance with MRI for several locoregional imaging features of rectal cancer. It may provide complementary information, particularly when MRI is contraindicated or inconclusive. In addition, when DLSCT is performed for systemic staging, it has the potential to provide both locoregional and distant disease assessment within a single examination. However, these findings do not establish equivalence to MRI or diagnostic accuracy against histopathology, and further prospective studies with pathological correlation are warranted.