Aims <p>Potassium (K) deficiency is a major global constraint compromising crop growth, stress tolerance, quality, and yield. Nevertheless, the biological basis of adaptive responses to K deprivation remains elusive in crops with complex genomic architectures, particularly allotetraploid rapeseed (<i>Brassica napus</i> L.).</p> Methods <p>Integrated shoot performance and root architecture, leaf ultrastructure, ionomics, phytohormone profiles, comparative genome-wide gene expression profiling, high-resolution metabolite fingerprints, and weighted network analysis were combined to dissect the morpho-physiological, molecular, and metabolic responses of rapeseed plants to K deficiency.</p> Results <p>In this study, our investigation revealed that chlorotic leaf performance, decreased photosynthesis capacity, retarded root growth, excessive reactive oxygen species and abscisic acid accumulation, and disordered Fe and Mg ion homeostasis in the rapeseed plants grown under low K stress. Gene ontology and Kyoto Encyclopedia of Genes and Genomes of differentially expressed genes and differentially abundant metabolites coordinated reprogramming of critical pathways involving ion uptake and transport, starch and sucrose metabolism, amino acid metabolism, and phytohormone signaling transduction. Core K transporters from the multicopy family genes, alongside novel candidate genes (e.g. those encoding cuticular wax biosynthesis) and metabolites (e.g. lipids) were proposed as promising regulators for the adaptative responses of rapeseed plants to K deficiency.</p> Conclusions <p>These findings establish a holistic framework connecting physiological manifestations with transcriptional-metabolic reprogramming under K deficiency, while proposing strategic targets for precision K management in rapeseed cultivation.</p>