Background: Locally advanced rectal cancer (LARC) treatment selection between neoadjuvant intensified chemotherapy (NIC) and standard chemoradiotherapy (CRT) lacks validated predictive biomarkers. Interleukin-1β (IL1B) polymorphisms modulate IL-1β expression and may influence treatment-specific outcomes through tumor microenvironment remodeling. Methods: In this prospective, single-center, randomized, open-label noninferiority trial (NCT05378919), 150 patients with stage II–III rectal cancer were randomized 1:1 to NIC (FOLFOX4, 8 cycles; n=68) or CRT (50 Gy/25 fractions with concurrent fluoropyrimidine; n=82). The primary endpoint was 5-year disease-free survival (DFS; noninferiority margin HR ≤1.29, NIC vs. CRT). IL1B rs1143634 and rs16944 were genotyped by TaqMan assay. Treatment–genotype interactions were assessed using Cox regression with multiplicative interaction terms. Results: Noninferiority was not demonstrated: 5-year DFS was 54.4% (NIC) versus 68.1% (CRT) in the ITT population (HR 1.52, 95% CI 0.90–2.56; p=0.114), with the upper CI bound exceeding the prespecified margin of 1.29. Per-protocol DFS was significantly inferior for NIC (62.5% vs. 82.1%; HR 2.27, 95% CI 1.06–4.76; p=0.029). Overall survival and cancer-specific survival (CSS) did not differ significantly between arms. A statistically significant treatment–genotype interaction was identified for rs1143634 in CSS (interaction p<0.001): T-allele carriage (CT+TT) was associated with inferior CSS exclusively in the NIC arm (HR 4.84, 95% CI 1.04–22.43; p=0.044), with no prognostic effect in the CRT arm (HR 0.69; p=0.494). A DFS interaction was also significant (p=0.047). rs16944 showed a borderline CSS interaction (p=0.082). Conclusions: NIC (FOLFOX4) did not achieve DFS noninferiority versus CRT in unselected LARC patients. IL1B rs1143634 demonstrates a treatment-arm-specific prognostic effect on cancer-specific survival, with a highly significant treatment–genotype interaction (p<0.001) identified within a prospective randomized design with prespecified translational hypotheses, satisfying the formal criterion for predictive biomarker classification. Prospective validation in an adequately powered, biomarker-stratified trial is warranted.