The broiler industry has come under sustained pressure from welfare organizations to improve broiler welfare. Breed and stocking density are important factors for broiler welfare and are often specified as criteria for higher welfare systems. However, it remains unknown how slower-growing broilers respond to a reduction in stocking density with regard to their behaviour, and whether this response differs from fast-growing broilers. Therefore, we compared fast- (Ross 308) and slower-growing broilers (Ranger Classic) housed at 4 different stocking densities (24, 30, 36 and 42 kg/m2, based on slaughter weight) with regard to their behaviour, responses to behavioural tests (i.e., novel object (NO), human approach (HA) and free-space (FS) test) and enrichment use at 4 target body weights (TBW's, 0.4, 1.1, 1.7 and 2.1 kg). The experiment had a 2 × 4 factorial design with 4 replicates (pens) per treatment (total of 32 pens). Thinning (15%) was done in a 50/50 male/female ratio at 38 (Ross 308) and 44 (Ranger Classic) days of age (estimated body weight of 2.2 kg). We hypothesized that slower-growing broilers would respond more strongly to a reduction in stocking density with regard to their behaviour. As slower-growing broilers are more active and show more comfort and foraging behaviours, it is expected that with increasing space they will respond with a larger increase in these types of behaviours compared to fast-growing broilers. Contrary to our hypothesis, hardly any interactions between breed and stocking density were found, indicating that fast- and slower-growing broilers showed similar responses to a reduction in stocking density. Broilers housed at lower stocking densities (24 and/or 30 kg/m2) showed more comfort and foraging behaviour, and showed more frolicking, running and sparring in the FS test compared to those housed at higher stocking densities (36 and/or 42 kg/m2). Slower-growing broilers showed less ingestion, more locomotion, standing, comfort and foraging behaviour, made better use of the enrichment by sitting on the bale, and more slower-growing broilers approached a human and NO compared to fast-growing broilers. In conclusion, reducing stocking density positively affected performance of comfort, foraging and play behaviours, indicating improved welfare. Slower-growing broilers showed more locomotive, comfort and foraging behaviours, less fear and a better use of enrichments, suggesting improved welfare compared to fast-growing broilers. Thus, reducing stocking density and using slower-growing broilers would benefit broiler welfare, where combining both would further improve broiler welfare with regard to their behaviour.