Socialisation is a process in which animals interact with unfamiliar conspecifics, that allows them to develop their social abilities. Socialisation has been proposed as a method in pig husbandry to increase piglets’ social skills and reduce conspecific aggression, which is a major welfare issue in pig husbandry. Socialisation, i.e. co-mingling, has been studied in research settings, with positive outcomes, but research under commercial conditions is lacking. The aim of this study was to obtain behavioural, stress coping and performance parameters of piglets socialised at different ages under intensive commercial conditions. Piglets were either not socialised (CON; n = 12 litters) or socialised at 7 d of age (M7; n = 20 litters) or 14 d of age (M14; n = 20 litters) by removing the separating barrier between two adjacent pens. At weaning (day 25), all piglets were regrouped with unfamiliar piglets from the same treatment. Behavioural observations (instantaneous scan sampling) took place after socialisation at day 7 and day 14, and at day 21. Skin lesions were counted pre- and post-mixing, including weaning. At the same time saliva was sampled to measure cortisol concentration. Weight was recorded weekly. Social behaviour did not differ between treatments at day 7 and 21, but at day 14 piglets from the socialised group (M7 and M14) showed a three-fold increase in play behaviour compared to the control group (CON). M7 and M14 did not differ in the amount of social behaviour but socialisation treatments showed more aggression than CON at day 14. During lactation, skin lesions increased after socialisation in M7 piglets whereas they did not in the M14 group. After weaning, skin lesions increased in the CON group but did not in the M7 and M14. Socialised piglets were lighter than the CON group during lactation, but treatments did not differ in their average daily gain (ADG) as socialised piglets had a smaller growth reduction after weaning. Salivary cortisol did not differ between treatments. The present study shows a positive effect of socialisation on social behaviour during lactation with only short lived aggression just after mixing and no impact for piglet’s growth.