Aiming to improve the housing conditions of stallions, we tested the “social box” (SB), which allows closer physical contact between stallions housed individually in internal stables. The partition of the SB comprised vertical bars that allowed the horses to pass their head, neck and legs into the adjacent box stall. Eight pairs of stallions were filmed over a 24 h period in the SB and in their usual box stables, “conventional boxes” (CB), which strongly restrict tactile contact. We investigated the effect of housing in the SB on horses’ behaviour and the number and characteristics of injuries. The total duration of active social interactions was significantly higher in the SB than in the CB (51 min vs. 5 min). Positive interactions accounted for about 71% of the total duration of interactions in SB and CB. The stallions interacted more often in the SB than in the CB (113.5 vs. 23.8 social interaction sequences over 24 h). No grievous injuries were recorded. The social box appears to be a suitable and innovative solution to give singly housed adult stallions the possibility of having closer physical interactions in a safe way, limiting the risk of injuries they could inflict or be exposed to.