The traditional Iberian pig production system is characterized by a final open-air fattening period (montanera), in which the animals are fed acorns and grass, preceded by a growing period where the feeding is restricted to avoid undesirable weight gain. New growing diets based on olive agro-industrial by-products could be an alternative to avoid this restriction. The objective of the current study (of TREASURE project) was to analyse the effect of two growing alternative diets on the gut microbiota composition from faecal samples collected before and after montanera. Three diets, one incorporating dry olive pulp in the feed (DD), one incorporating olive cake in wet form (WD) and a control diet (CD) were supplied to 45 animals (15 per diet) during the growing period (45 to 95 kg of body weight). The gut microbiota composition of each individual was evaluated at two time points: before transition to montanera (95 kg) and at slaughter (160 kg). Microbiota analyses were performed by re-sequencing the bacterial 16S gene (V3-V4) in an Illumina MiSeq. Bioinformatics analyses were performed by using Qiime’s open-reference subsampled OTU calling approach. The effect of diets on microbiota composition and diversity was evaluated using the Vegan package in R. Bray-Curtis distances, NMDS and PERMANOVA analyses showed significant effects on microbiota composition. WD caused an increase in microbiota diversity. In the second sampling point, differences in composition and diversity could be also observed after acorn supplementation. Funded by European Union’s H2020 RIA program (Grant agreement no. 634476).