The present study investigated the effects of dietary supplementation of a seaweed extract (SWE) to sows and weaned pigs on post-weaning growth performance, intestinal morphology, intestinal microflora, volatile fatty acid concentrations and immune status of pigs at days 11 and 117 post-weaning. Gestating sows (n 20) were supplemented with a SWE (0 v. 10·0 g/d) from day 107 of gestation until weaning (day 26). At weaning, pigs (four pigs per sow) were divided into two groups based on sow diet during lactation and supplemented with a SWE (0 v. 2·8 g/kg diet), resulting in four treatment groups: (1) BB (basal sows-basal pigs); (2) BS (basal sows-treated pigs); (3) SB (treated sows-basal pigs); (4) SS (treated sows-treated pigs). Pigs weaned from SWE-supplemented sows had a higher average daily gain (ADG) between days 0 and 21 (P < 0·05) post-weaning compared with pigs weaned from non-SWE-supplemented sows. Pigs offered post-weaning diets (PW) containing SWE had decreased colonic Escherichia coli populations on day 11 (P < 0·01) and decreased colonic Enterobacteriaceae numbers on day 117 (P < 0·05). Pigs offered PW containing SWE had a greater mRNA abundance of MUC2 in the colon at day 11 post-weaning (P < 0·05) compared with pigs offered unsupplemented diets. In conclusion, these results demonstrate that SWE supplementation post-weaning provides a dietary means to improve gut health and to enhance growth performance in starter pigs. Dietary SWE supplementation increased ADG during the grower-finisher (GF) phases. However, there was no growth response to SWE inclusion in GF diets when pigs were weaned from SWE-supplemented sows.