The 2003–04 Phoenix Suns season was the 36th for the Phoenix Suns of the National Basketball Association. After advancing to the playoffs the previous season, the Suns started off to a disappointing start under head coach Frank Johnson. With the team at 8–13, team management elected to turn to assistant coach Mike D'Antoni to take over for Johnson. Under D'Antoni, the Suns would lose 40 of their next 61 games and miss the playoffs, finishing sixth in the Pacific division with a 29–53 regular season record, the first time since the 1987–88 season the Suns recorded 50 losses or more. The Suns played their home games at America West Arena.
Before the halfway mark of the season, the Suns sent starting point guard Stephon Marbury along with Anfernee Hardaway to the New York Knicks for Antonio McDyess, and a lack of offense was felt the rest of the season. The Suns found the injury bug, with reigning Rookie of the Year Amar'e Stoudemire missing nearly 30 games (and the Suns his 20 points and nine rebounds per game), and oft-injured Hardaway and Tom Gugliotta sidelined once again with injuries that had plagued the careers of both while playing for the Suns. Power forward Shawn Marion did not repeat as an All-Star, despite ending the season averaging 19 points and 9.3 rebounds per game and finishing second in the league in steals per game. Joe Johnson had a breakthrough year in his third season NBA season, leading the league in minutes played and providing the Suns nearly 17 points a game. The Suns also got younger, at quicker, at the point guard position when Leandro Barbosa was drafted. "The Brazilian Blur" averaged eight points per contest his rookie season.
By trading Marbury, the Suns were without a point guard to lead an improving young core of Stoudemire, Marion, Johnson and Barbosa, all 25 years of age or younger. By shedding the injury-riddled Hardaway and Gugliotta, as well as Marbury, the Suns ended the season with a need for a club leader and money at their disposal. Following the season, McDyess signed as a free agent with the Detroit Pistons.