Sport is nowadays perceived as beneficial for children with haemophilia, as good muscle strength supports joints and may reduce bleed frequency; by contrast psychological benefits are less known. This study introduces the impact of sport on health-related quality of life (HRQoL) and physical performance in children with haemophilia. A cross-sectional, multi-site, study of boys aged 6–17 years with haemophilia A or B of any severity, current or past inhibitor, which assessed physical performance, sporting activity and HRQoL using age appropriate questionnaires including KINDL, Haemo-QoL and HEP-Test-Q. Eighty-four haemophilic boys (23 mild, 19 moderate, 42 severe) with a mean age of 11.52 years (SD = 3.4) were enrolled from two haemophilia centres in the United Kingdom. 28.4% were overweight/obese according to their BMI/age and had a good orthopaedic status (M = 1.55, SD = 3.3). Boys watching < 1–2 h of TV/PC/day had fewer days lost (M = 3, SD = 3.2) than those with a more sedentary lifestyle (M = 9.40, SD = 7.1) (P < 0.032). 90.5% participated in regular sporting activity; 79.9% at least twice a week. HRQoL in children was generally good, with highest impairments in boys aged 8–12 years. Boys aged 8–16 years reported good physical performance (M = 80.0, SD = 16.0) with highest impairments in the dimensions ‘endurance’ and ‘mobility’. Boys doing sport had a significant better physical performance and HRQoL than boys not doing sport. Sedentary life styles had a negative impact on the subjective physical performance and number of days lost of children. Encouraging haemophilic boys to participate in sport will have a direct impact on their overall HRQoL.