Age at weaning affects the behaviour of piglets weaned in conventional confined environments. The adaptation of piglets to this event, which exposes piglets to important stressors, has not been examined in detail in outdoor systems. The aim of this study was to compare the behaviour of piglets weaned at 3 or 4 weeks of age in an outdoor production system. Six replicates of four piglets born and raised outdoors, originated from different litters but previously acquainted, were weaned at the age of approximately 20 (D20) or 30 (D30) days in 36 m2 outdoor pens and offered high-quality diets. Their behaviour was recorded by direct visual observation during four consecutive days after weaning (days 1 to 4). Data were analysed by repeated measures ANOVA. An effect of weaning age was observed on feeding and rooting ( P < 0.01), and a day-by-weaning age interaction for escape attempts, vocalizing, walking and fighting ( P < 0.01), and playing ( P < 0.03). Compared to D30 piglets, D20 piglets showed a higher frequency of escape attempts on day 1, vocalized more during days 1 and 2, and walked more during days 1 to 3 ( P < 0.05). Feeding behaviour, on the other hand, was higher in D30 piglets on days 1 to 3 ( P < 0.05). On days 3 and 4, D30 piglets spent more time interacting with peers (playing and fighting; P < 0.05) and on day 4 were more active than D20 piglets ( P < 0.05). As previously reported in confined environments, age at weaning affected the behaviour of piglets raised on the outdoors system. Weaning appeared to be more stressful for the piglets at 3 than at 4 weeks of age. The impact of weaning age on welfare cannot be ignored, especially when outdoor breeding of pigs is proposed to address welfare concerns of the public.