The Arizona Diamondbacks (often shortened as the D-Backs) are an American professional baseball team based in Phoenix, Arizona. They were formed in 1998 and are based at Chase Field (formerly Bank One Ballpark), after five years of preparation under the leadership of Jerry Colangelo. The Diamondbacks won the 2001 World Series against the Yankees, becoming the fastest expansion team in Major League Baseball to win a championship, doing so in only the fourth season since their first in 1998. Financial difficulties later ensued, leading to the home field being renamed Chase Field in 2005, as a result of Bank One Corporation's merger with JPMorgan Chase & Co. After a lean period, the team won the National League West division in 2011. During the 2023 season, the team finished as a wild card; however, they would upset several teams en route to their second-ever World Series appearance. They would lose the 2023 World Series 4-1 to the Texas Rangers.

1940–1997: Bringing Major League Baseball to Arizona Baseball had a rich tradition in Arizona long before talk of bringing a big-league team even started. The state has been a frequent spring training site since 1946. With a large number of people relocating to the state from the Midwest and the Northeast, as well as from California, many teams (most notably the Chicago Cubs and the San Francisco Giants) have normally had large followings in Arizona. By the 1980s, the Phoenix metropolitan area's explosive growth since the end of World War II had made it the 14th-largest in the nation, with over two million residents. Phoenix proper had risen to the ninth largest city in the nation after being 99th in 1950. It was a foregone conclusion that the city would eventually get a major league team.

The first serious attempt to land an expansion team for the Phoenix area was mounted by and Martin Stone, owner of the Phoenix Firebirds, the city's triple-A minor league baseball team and the top affiliate of the San Francisco Giants. In the late 1980s Stone approached St. Louis (football) Cardinals owner Bill Bidwill about sharing a proposed 70,000 seat domed stadium in Phoenix that would have been the centerpiece of a major plan to redevelop downtown Phoenix. It would have been financed with $150 million in municipal bonds, which would have been repaid with the proceeds from skyboxes and luxury seats. It was taken for granted that a domed stadium was a must for a major-league team to be a viable venture in the Phoenix area. Phoenix is by far the hottest major city in North America; the average high temperature during baseball's regular season is , and game-time temperatures well above are very common during the summer.

Bidwill, with plans already in the works to leave St. Louis, opted instead to sign a long-term lease with Arizona State University to use its Sun Devil Stadium as the home of his soon-to-be Arizona-based NFL franchise. Since baseball-only stadiums were not seen as fiscally viable during that era, Stone pulled his support for the stadium. In 1989, Phoenix voters overwhelmingly rejected the issuance of the first $100 million in bonds for the proposed stadium. In response, the Arizona Legislature authorized Phoenix to bypass a referendum in order to build a stadium. It also authorized Maricopa County, home to Phoenix and most of its suburbs, to finance any proposed stadium with a 0.25 percent sales tax.

In the fall of 1993, Jerry Colangelo, majority owner of the Phoenix Suns, the area's NBA franchise, announced he was assembling an ownership group, "Arizona Baseball, Inc.", to apply for a Major League Baseball expansion team. This was after a great deal of lobbying by the Maricopa County Sports Authority, a local group formed to preserve Cactus League spring training in Arizona and eventually secure a Major League franchise for the state.

Colangelo's group was so confident that they would be awarded a franchise that they held a public contest to name the prospective team. As a demonstration of their certainty, they took out a full-page advertisement in the sports section of the February 13, 1995 edition of the Arizona Republic, the state's leading newspaper, to promote the naming contest. First prize was a pair of lifetime season tickets awarded to the person who submitted the winning entry. The winning choice was "Diamondbacks", after the Western diamondback, a rattlesnake native to the region known for injecting a large amount of venom when it strikes. In addition to a write-in option, the four provided choices that did not win were the Arizona "Coyotes", "Phoenix", "Rattlers", and "Scorpions".Arizona Baseball Inc. advertisement. Arizona Republic, 13 Feb. 1995, p. C10.

Colangelo's bid received strong support from one of his friends, Chicago White Sox and Chicago Bulls owner Jerry Reinsdorf, and media reports say that then-acting Commissioner of Baseball and Milwaukee Brewers founder Bud Selig was also a strong supporter of Colangelo's bid.AZStarNet.com Plans were also made for a new retractable-roof ballpark, Bank One Ballpark, nicknamed the BOB, (renamed in 2005 to Chase Field) to be built in an industrial/warehouse district on the southeast edge of downtown Phoenix, one block from the Suns' America West Arena (now Mortgage Matchup Center).

On March 9, 1995, Colangelo's group was awarded a franchise to begin play for the 1998 season. A $130 million franchise fee was paid to Major League Baseball. The Tampa Bay area was also granted a franchise, the Devil Rays (to be based in St. Petersburg), at the same time.

Arizona had originally been intended to join Tampa Bay in the American League. However, five American League teams had threatened to block the league assignments because of concerns that they would have additional games out of their time zone, causing early starts that would decrease revenue and TV ratings. Thus, on January 16, 1997, the Diamondbacks were officially voted into the National League while their expansion counterparts in Tampa Bay were voted into the American League. MLB reserved the right to unilaterally move either the Diamondbacks or Devil Rays to another league within the first five years of existence. After the 2001 season, with the possibility of the Twins and Expos being eliminated, it was possible the Diamondbacks would move to the American League. However, MLB's franchise contraction plans never materialized.

According to the original press release from Colangelo's group (which remained posted on the team website during the first few seasons) the chosen team colors were Arizona turquoise, copper, black and purple. "... Turquoise was chosen because the greenish-blue stone is indigenous to Arizona, copper because Arizona is one [of] the nation's top copper-producing states and purple because it has become a favorite color for Arizona sports fans, thanks to the success of the National Basketball Association's Phoenix Suns." The distinctive "A" logo and color scheme was developed by Campbell Fisher Ditko, a Phoenix-based graphic design firm (which also designed the now-iconic "Streaking Sun" logo for the Suns for the 1992 season).

In the earliest days, the Diamondbacks operated as a subsidiary of the Suns; several executives and managers with the Suns and America West Arena were brought over to the Diamondbacks in similar roles. As the expansion draft had yet to take place, there were no actual Diamondback players, so members of the Suns team, including team captain and three-time NBA All-Star point guard Kevin Johnson (who was actually drafted by the Oakland A's in 1986 as a shortstop; he chose to focus exclusively on his basketball career as a pro), Danny Ainge (who actually briefly played in Major League Baseball at the beginning of his career for the Toronto Blue Jays), Danny Manning and coach Paul Westphal, joined Colangelo and modeled the first prototype Diamondbacks uniforms, also designed by Campbell Fisher Ditko, at a publicity event in early November 1995. These uniforms and caps were immediately made available for sale to the public.

From the beginning, Colangelo wanted to market the Diamondbacks to a statewide fan base and not limit fan appeal to Phoenix and its suburbs. Colangelo decided to call the team the "Arizona Diamondbacks" rather than the "Phoenix Diamondbacks". Although every Major League Baseball team does cultivate fans from outside its immediate metropolitan area, and even though the greater Phoenix area has 2/3 of the statewide population, there are long-standing historic rivalries between Phoenix and Arizona's other cities, such as Tucson, which at times diminish statewide appeal and enthusiasm for a Phoenix-based team.

A series of team-sponsored fan motor coach trips from Tucson to Bank One Ballpark were inaugurated for the opening season and are still in operation to this day (it is now known as the "D-backs Express"). The Diamondbacks are also known for the "Hometown Tour", held in January, where selected players, management and broadcasters make public appearances, hold autograph signings, etc., in various locations around Phoenix and Tucson, as well as many small and mid-sized towns in other areas of Arizona.

Despite their early efforts to appeal to the entire state, in 2009 the Diamondbacks moved their Triple-A Pacific Coast League farm team, the Tucson Sidewinders, to Reno, where they are now known as the Aces. In 2011, they also moved their spring training base from Tucson to a new stadium near the Phoenix suburb of Scottsdale.

Two years before their first opening day, Colangelo hired the then-recently fired Buck Showalter, the American League Manager of the Year in 1994 with the New York Yankees, as the D-Backs' first manager.

Their lower level Minor League Baseball teams began play in 1996; the expansion draft was held in November 1997. The Diamondbacks would replace the above-mentioned Firebirds as the main baseball team in Phoenix; they would move south to Tucson and become the Sidewinders, serving as the AAA club for the new team.

1998–2002: Expansion days, World Series surprise

The Diamondbacks' first major league game was played against the Colorado Rockies on March 31, 1998, at Bank One Ballpark before a standing-room only crowd of 50,179. Tickets had gone on sale on January 10 and sold out before lunch. The Rockies won, 9–2, with Andy Benes on the mound for the Diamondbacks, and Travis Lee being the first Diamondbacks player to get a hit (a single in his first MLB at bat), then became the first Diamondback to score, homer and drive in a run in his second MLB plate appearance.

In their first five seasons of existence, the Diamondbacks won three division titles (1999, 2001, 2002) a National League pennant (2001) and a World Series championship (2001). In 1999, Arizona won 100 games in only its second season to win the NL West title. They lost to the New York Mets in four games in the NLDS.

Colangelo fired Showalter after a relatively disappointing 2000 season, and replaced him with Bob Brenly, the former Giants catcher and coach, who had up to that point been working as a color analyst on Diamondbacks television broadcasts. In 2001, the team was led by two of the most dominant pitchers in all of baseball: Randy Johnson and Curt Schilling. Arizona had postseason victories over the St. Louis Cardinals (3–2 in the NLDS) and the Atlanta Braves (4–1 in the NLCS) to advance to the World Series where, in the wake of the September 11 terrorist attacks in New York City, they beat the three-time reigning champions, the New York Yankees, 4–3, to become the youngest expansion franchise to win the World Series (in just their fourth season of play). The previous mark was held by the Florida Marlins. This was the first time since that the home team won all seven games of a World Series, and the Diamondbacks were the first National League team to prevail in such a World Series.