A 2x2 factorial experiment (ten sows per treatment) was conducted to investigate the effect of maternal dietary supplementation with a seaweed extract (SWE; 0 v. 10·0 g/d) and fish oil (FO; 0 v. 100 g/d) inclusion from day 109 of gestation until weaning (day 26) on pig performance post-weaning (PW) and intestinal morphology, selected microflora and immune status of pigs 9 d PW. The SWE contained laminarin (10 %), fucoidan (8 %) and ash (82 %) and the FO contained 40 % EPA and 25 % DHA. Pigs weaned from SWE-supplemented sows had higher daily gain (P=0·063) between days 0 and 21 PW and pigs weaned from FO-supplemented sows had higher daily gain (P<0·05) and gain to feed ratio (P<0·01) between days 7 and 14 PW. There was an interaction between maternal SWE and FO supplementation on caecal Escherichia coli numbers (P<0·05) and the villous height to crypt depth ratio in the ileum (P<0·01) and jejunum (P<0·05) in pigs 9 d PW. Pigs weaned from SWE-supplemented sows had lower caecal E. coli and a higher villous height to crypt depth ratio in the ileum and jejunum compared with non-SWE-supplemented sows (P<0·05). There was no effect of SWE on E. coli numbers and villous height to crypt depth ratio with FO inclusion. Maternal FO supplementation induced an increase in colonic mRNA abundance of IL-1α and IL-6 (P<0·05), while SWE supplementation induced an increase in ileal TNF-α (P<0·01) and colonic TFF3 mRNA expression (P<0·05). In conclusion, these results demonstrate that SWE and FO supplementation to the maternal diet influenced the gastrointestinal environment and performance of the weaned pig.