The poultry industry requires sustainable strategies to improve disease resistance and reduce reliance on antibiotics. β-glucans, naturally occurring polysaccharides widely used as feed additives, have attracted considerable attention as immunomodulatory compounds that can induce trained immunity. This process involves the long-term functional reprogramming of innate immune cells, resulting in enhanced responses to subsequent challenges that are largely independent of classical adaptive immune memory. In chickens, β-glucan exposure has been shown to prime heterophils, monocytes, and macrophages through pattern-recognition receptor-mediated signalling pathways, leading to metabolic and epigenetic reprogramming associated with increased cytokine production, phagocytic activity, oxidative burst, and antimicrobial functions. Emerging evidence further suggests that β-glucans influence gut microbiota composition, stimulate gut-associated lymphoid tissues, and help maintain intestinal barrier function, thereby contributing to immune homeostasis. In addition, early-life exposure to β-glucans may promote long-lasting alterations in myeloid cell function consistent with trained immunity. Overall, these immunological effects suggest that β-glucan-induced trained immunity may help improve disease resistance, vaccine-induced protection, health and productivity, and reduce antibiotic use in poultry production. This review summarises current knowledge on the cellular and molecular mechanisms by which β-glucans induce trained-immunity-like responses and reprogramme innate immune cells and discusses its implications for poultry health.