The purpose of this study was to examine the seasonal changes in body composition, nutrition, and upper body strength in professional Australian Football (AF) players. A prospective longitudinal study examined changes in anthropometry (body mass, fat-free soft tissue mass (FFSTM) and fat mass (FM)) via dual-energy X-ray absorptiometry (DXA) five times during an AF season (start-preseason, mid-preseason, start-inseason, mid-inseason, end-inseason) in 45 professional AF players. Dietary intakes and strength (bench press and bench pull) were also assessed at these time points. Players were categorised as experienced (>4 y experience, N=23) or inexperienced (<4 y experience, N=22). FM decreased during the preseason but was stable through the inseason for both groups. %FFSTM was increased during the preseason and remained constant thereafter. Upper body strength increased during the preseason and was maintained during the inseason. Changes in upper body FFSTM were related to changes in UB strength performance (r = 0.37-0.40). Total energy and carbohydrate intakes were similar between the experienced and inexperienced players during the season, but there was a greater ratio of dietary fat intake at the start-preseason point, and an increased alcohol, reduced protein and increased total energy intake at the end of the season. The inexperienced players consumed more fat at the start of season and less total protein during the season compared to the experienced players. Coaches should also be aware that it can take >1 y to develop the appropriate levels of FFSTM in young players and take a long-term view when developing the physical and performance abilities of inexperienced players.