The confinement of gestating sows and gilts has been a welfare concern in commercial pig production worldwide, and group housing systems have been proposed as an alternative. However, previous studies have provided limited information on sow behaviour and welfare, particularly under commercial confinement and group housing systems, and no comparable research has been conducted in Japan. This study investigated how gestation stalls and group housing systems affect the behaviour and welfare of pregnant sows on a commercial farm. Gestating sows were housed in either gestation stalls (GS; 29 sows) or a group housing system (GHS; 30 sows), and their behaviour and posture were recorded. Sows in the GS group spent more time exhibiting abnormal behaviours and standing (all P < 0.05), whereas sows in the GHS group spent more time resting, engaging in social behaviour, and lying laterally (all P < 0.01). These results suggest that sows housed in groups exhibit natural behaviour, low stress levels, and high welfare levels. These findings provide foundational knowledge about the behaviour of pregnant sows in Japanese commercial housing environments.