The 2004 Arizona Diamondbacks season was the franchise's 7th season in Major League Baseball and their 7th season at Bank One Ballpark in Phoenix, Arizona, as members of the National League West. They looked to improve on their 84–78 record from 2003. The D-backs hoped to contend for a postseason berth in what was a weaker National League West than in years past. However, just three years after winning a World Series title, the Diamondbacks instead finished the season with a record of 51–111, the worst record by any National League team since the 1965 Mets won one fewer game. The one highlight of a disastrous season was when Randy Johnson pitched a perfect game on May 18, 2004. The franchise record would not be threatened until 2021, when they finished 52–110. Despite the 110-loss season in 2021, the Diamondbacks didn't earn the first overall pick in the 2022 MLB Draft, as the Baltimore Orioles finished with a 52–110 record, but were worse due to records against American League opponents.

Offseason

Regular season

Opening Day lineup

Season standings

National League West

Record vs. opponents

Notable transactions

Roster

Player stats

Batting

Starters by position

''Note: Pos = Position; G = Games played; AB = At bats; H = Hits; Avg. = Batting average; HR = Home runs; RBI = Runs batted in''

Other batters

''Note: G = Games played; AB = At bats; H = Hits; Avg. = Batting average; HR = Home runs; RBI = Runs batted in''

Pitching

Starting pitchers

Note: G = Games pitched; GS = Games started; IP = Innings pitched; W = Wins; L = Losses; ERA = Earned run average; SO = Strikeouts

Other pitchers

Note: G = Games pitched; IP = Innings pitched; W = Wins; L = Losses; ERA = Earned run average; SO = Strikeouts

Relief pitchers

Note: G = Games pitched; W = Wins; L = Losses; SV = Saves; ERA = Earned run average; SO = Strikeouts

Farm system