Providing a variety of elements in the rearing environment may offer a promising way to transition conventional broiler production towards more animal-friendly production systems. This study aimed to investigate the effect of a complex and structured rearing environment on broiler welfare by comparing three complex enriched rooms (E) with visual cues and multiple elements for resting or exploration, to three control rooms (C) with minimal enrichments. Each room housed 2450 Redbro chickens and received natural light. We evaluated how the rearing environment influenced chicken behavior, emotional state, and the cecal microbiota composition at the end of the rearing period. E chickens stood, foraged and moved more often than C chickens (e.g. 38% vs. 19% walking; 4% vs. 1% foraging, P < 0.001), while dustbathing frequency did not differ. In group-based tests, E chickens were more likely to approach and peck at a novel object or human (P < 0.05), suggesting reduced fearfulness and increased curiosity. In the detour test, E chickens exited the U-shaped area more frequently (P < 0.001) and vocalized less (P = 0.004), indicating greater exploratory motivation and possibly better spatial cognition. Microbiota analysis revealed no differences in alpha diversity, but beta diversity differed significantly between treatments (P < 0.001). E chickens had higher relative abundances of Bacteroidota, while C chickens had more Bacillota. Overall, the complex environment promoted natural behaviors, reduced emotional reactivity, and altered gut microbiota composition, supporting its potential to improve broiler welfare under semi-commercial conditions.