The prognosis of patients with locally advanced rectal cancer (LARC) has improved with the adoption of a multidisciplinary treatment approach combining neoadjuvant chemoradiotherapy (nCRT) and total mesorectal excision (TME). Developing real-time, sensitive biomarkers to monitor systemic changes during nCRT is of paramount importance. Although the association between albumin-derived neutrophil-to-lymphocyte ratio (Alb-dNLR) and prognosis in various cancers is established, its prognostic value in LARC patients undergoing nCRT is not well-studied. This study enrolled a cohort of 618 LARC patients, stratifying them into two groups according to their change in Alb-dNLR (∆Alb-dNLR) values, using an optimal cut-off point: a low ∆Alb-dNLR group (≤ 0.90) and a high ∆Alb-dNLR group (> 0.90). The prognostic significance of ∆Alb-dNLR was evaluated using a Cox proportional hazards model. The 5-year overall survival (OS) rates were 75.2% in the low ∆Alb-dNLR group (≤ 0.90) and 85.9% in the high ∆Alb-dNLR group (>0.90) (P < 0.001). The 5-year disease-free survival (DFS) rates were 71.2% and 80.6%, respectively (P = 0.016). Multivariate analyses demonstrated that both ∆Alb-dNLR and pre-Alb-dNLR were independent prognostic factors for OS (P ≤ 0.001), while ∆Alb-dNLR was demonstrated as an independent prognostic factor for DFS (P = 0.016). A predictive nomogram, incorporating the ∆Alb-dNLR subgroup, demonstrated enhanced performance (concordance index [C-index] of 0.720 for OS and 0.690 for DFS) compared to the pre-treatment Alb-dNLR subgroup (C-index of 0.700 for OS and of 0.680 for DFS). Therefore, ∆Alb-dNLR shows significant potential as a usable and prognostic biomarker for predicting OS and DFS in LARC patients undergoing nCRT.