In the sports of water polo and soccer, the effects of ball inflation pressure on head impact forces have received little attention. Determining this relationship may help minimize the results of ball-head contact in these sports and become important for preventing concussion. We studied the effect of ball inflation pressure (35-117 kPa; 5-17 psi) and headgear on head linear impact acceleration and angular velocity using an instrumented, anthropomorphic, test dummy head. Water polo and soccer balls were shot at the forehead at 17.4 to 24.6 m/s (39 to 55 mph), the speeds of the fastest shots in elite water polo, as well as speeds which are typical for heading from goalie punts in competitive soccer. Overinflating the ball beyond the recommended pressure for each sport – 90 to 117 kPa (13 to 17 psi) for water polo, 55 to 76 kPa (8 to 11 psi) for soccer - significantly increased linear impact acceleration by 7% for both water polo and soccer balls (p < 0.001). Underinflating the ball from 90 to 69 kPa (13 to 10 psi) for water polo and 55 to 34 kPa (8 to 5 psi) for soccer significantly decreased linear impact acceleration by 8.5% for water polo and 13.5% for soccer (p < 0.001). Headgear is shown to reduce impact forces by 22% and angular velocity by 23.5%. These results suggest that appropriately inflating the ball and/or wearing padded headgear may help reduce the prevalence of brain trauma in water polo and soccer.