This experiment was conducted to investigate the effects of a purified fibre‐mixture (FM, 50% guar gum, 50% cellulose) supplementation of gestation diet on the immunity, faecal microbial composition and reproductive performance of sows. A day after breeding, 68 multiparous sows were randomly allocated to receive treatment with a control (CON) diet or a diet containing 3% FM (FM diet). Results showed the FM diet to be associated with a significant increase in the number of live‐born piglets relative to CON (13.65 vs. 12.47, p < .05). In addition, this FM diet coincided with significantly increased faecal concentrations of butyrate on day 30 and propionate on day 100 (p < .05), with trends towards increased propionate on day 30 and increased short‐chain fatty acids (SCFAs) on days 30 and 110 (p < .1). Meanwhile, FM addition markedly increased the abundance of representative SCFAs producing‐related genera as Roseburia on days 30 and 110 (p < .05), Eubacterium‐hallii‐group on days 30 and 110 (p < .05), and Bacteroides on day 110 of gestation (p < .05). The serotonin concentration on day 110 of gestation had increased (p < .05) and that on day 30 of gestation (p < .1) exhibited a tendency to increase with the FM‐supplemented diet in comparison with the CON. Besides, FM supplementation caused an increase in serum interleukin‐10 concentrations on days 30 (p < .05) and 110 of gestation (p < .1), and a decrease in interferon‐γ concentration on day 30 of gestation (p < .05). Together these results indicated that purified FM was able to improve sow reproductive performance through a mechanism potentially linked with a bias towards type‐2 helper T‐cell differentiation that supported embryonic survival and thereby improve reproductive yields. Changes in metabolites produced by the intestinal microbiome may thus have an impact on host immunity and reproductive performance.