The objective of this study was to evaluate the impact of chronic heat stress (HS) on microbiota composition of gestating sows and of their female offspring. There were two environmental conditions: thermoneutral (TN; cyclic 18 to 24 °C) and HS (cyclic 28 to 34 °C). Pregnant sows were exposed to either TN (PTN; n=12) or HS (PHS; n=12) conditions from d 9 to 109 of gestation. During the finishing period, two female offspring per sow were allotted to either a growing TN (GTN; n=24) or HS (GHS; n=24) environment. All pigs were fed the same diet all throughout the experiment. From 75 to 140 d of age, GTN pigs (n=24) were in TN conditions, while GHS pigs (n=24) were in TN conditions from 75 to 81 d of age and in GHS conditions from 82 to 140 d of age. Spot faecal samples were collected on d 4 and d 105 of gestation in the sows, and on 78 and 136 d of age in the growing pigs for 16S rRNA analyses of microbiota composition. In both sows and offspring, exposure to HS increased rectal (+0.16 and +0.30 °C, respectively; P<0.01) and skin (+2.44 and +1.61 °C respectively; P<0.01) temperatures. Firmicutes and Bacteroidetes were the most dominant phyla (~98%) in the samples collected pre-challenge regardless of physiological stage but sows had higher percentage of Firmicutes (+20.4%; P<0.01) and lower percentage of Bacteroidetes (-20.2%; P<0.01) than the growing pigs. Using a permutational multivariate analysis of variance on the environment effect on microbiota, chronic HS exposure did not significantly affect sow microbiota at d 105 (P=0.120). Microbiota from the pigs born from PHS sows was resistant to a further heat stress during the finishing (P=0.167), while microbiota of pigs born from the PTN sows were strongly affected (P=0.027). Therefore, the prenatal environment of the growing pigs affect how their microbial composition change upon exposure to a thermal stress during their postnatal life. The difference in the effect of heat exposure on gut microbiota of sows and of growing pigs may be related to their microbial composition or to the duration of their thermal stress exposure.