Formed 2003

Competed in the National Soccer League for one season in 2003-04 after taking the place vacated by Adelaide City Force. They then went on to become founding members of the A-League in 2005.

In the first season of the A-League they were the Minor Premiers finishing 7 points ahead of the pack but could only manage third place in the finals. The next season, 2006-07 despite only coming 2nd in the league, they went one better than the year before and made the Grand Final, however they lost badly, 6-0 to winners Melbourne Victory. Season 3 (2007-08) was a disappointment as Adelaide finished a lowly 6th.

Season 4 (2008–09) began full of hope as Adelaide United strengthened their squad. During the season Adelaide United passed the one millionth spectator milestone of total crowds since the inception of the A-League and are only the third A-League club to do so thus far, the other two being Melbourne Victory and Sydney FC. By the end of the regular, Adelaide had finished 2nd being equal to Melbourne Victory on 38 points on the ladder and also being equal on Goal Difference. The Championship came down to the fact that Melbourne had scored more goals. In the finals where they lost their Semi-Final 6–0 over two legs against Melbourne but then went on to beat Queensland Roar 1-0 in the Qualifying Final to go through to the Grand Final and face Melbourne again. The 2008-09 Grand Final was a much closer affair between these two teams unlike the Grand Final of 2 years earlier and it ended 1-0 to Melbourne Victory.