Increasing environmental complexity, i.e. by providing environmental enrichment, has been suggested as a way to improve broiler chicken welfare. One issue affecting broiler welfare is the level of fearfulness of the flock. Exaggerated fear responses may cause smothering and mortality. Environmental complexity has been found to reduce fearfulness in several species. It is known to promote the development of cognitive functions, including learning and memory. The aim of this study was to test the hypothesis that housing environments of different complexity, due to environmental enrichment or manipulation of standard resources, have an effect on learning ability and fearfulness in broilers. Fifty-eight pens with approximately 500 broilers in each (Ross 308), corresponding to a stocking density of 40 kg/m², were used. Commercial feed and water were provided ad libitum, and wood shavings were used as litter material. Five environmental enrichment treatments (roughage, vertical panels, straw bales, elevated platforms at different heights), three treatments where the standard resources had been manipulated (increased distances between feed and water, lower stocking density) and one control group were randomly assigned to each pen. At 21 d, 10 broilers from each pen with treatments straw bales, elevated platforms, lower stocking density and control group were tested in a passive avoidance-learning task. At 31 d, 30 broilers from each pen from all treatment groups were subjected to a tonic immobility test. There was a tendency for broilers housed with access to straw bales to have better performance in a passive avoidance-learning task compared to broilers in the control group (F_3,92 = 2.22; P = 0.091). In addition, chicks housed with access to 30 cm elevated platforms displayed shorter duration of tonic immobility compared to chicks housed with increased distance between feed and water (F_8,49 = 2.22; P = 0.041; Tukey's test P = 0.038), suggesting decreased fearfulness. The results show that increasing environmental complexity can have positive effects on decreasing fearfulness and promoting learning ability in broiler chickens, improving their welfare.