The Philadelphia Athletics were a Major League Baseball team that played in Philadelphia from 1901 to 1954, when they moved to Kansas City, Missouri, and became the Kansas City Athletics. After another move to Oakland, California in 1967, they became the Oakland Athletics. In 2025, the team moved to West Sacramento, California as the Athletics, and plans ultimately to move to the Las Vegas metropolitan area.

The Philadelphia Athletics had an overall win–loss record of 3,886–4,248–79 (.478) during their 54 years in Philadelphia. Eight former Philadelphia Athletics players were elected to the National Baseball Hall of Fame.

The Western League was renamed the American League in 1900 by league president Bancroft (Ban) Johnson and declared itself the second major league in 1901. Johnson created new franchises in the east and eliminated some franchises in the west. Philadelphia was given a new franchise to compete with the National League's Philadelphia Phillies.

Former catcher Connie Mack was recruited to manage the club. Mack in turn persuaded Phillies minority owner Ben Shibe as well as others to invest in the team, which would be called the Philadelphia Athletics, a name taken from the Athletic Base Ball Club of Philadelphia, which had been a founding member of the NL in 1876 but had folded after only one season. Mack himself bought a 25% interest, while the remaining 25% was sold to Philadelphia sportswriters Sam Jones and Frank Hough.

The new league recruited many of its players from the existing National League, persuading them to "jump" to the American League in defiance of their contracts. One of the players who jumped to the new league was second baseman Nap Lajoie, formerly of the crosstown Phillies. He won the A.L.'s first batting title with a .426 batting average, still a league record. The Athletics and the American League received a setback when, on April 21, 1902, the Pennsylvania Supreme Court invalidated Lajoie's contract with the Athletics, and ordered him back to the Phillies. This order, though, was only enforceable in the Commonwealth of Pennsylvania. Lajoie was sold to Cleveland, but was kept out of road games in Philadelphia until the National Agreement was signed between the two leagues in 1903.

Columbia Park was the Athletics first home. They played there from their founding in 1901 through the 1908 season, and it was the venue of their two home games in the 1905 World Series.

In the early years, the Athletics established themselves as one of the dominant teams in the new league, winning the A.L. pennant six times (1902, 1905, 1910, 1911, 1913, and 1914), and winning the World Series in 1910, 1911, and 1913. They won over 100 games in 1910 and 1911, and 99 games in 1914. The team was known for its "$100,000 infield," consisting of Stuffy McInnis (first base), Eddie Collins (second base), Jack Barry (shortstop), and Frank "Home Run" Baker (third base) as well as pitchers Eddie Plank and Chief Bender. Rube Waddell was also a major pitching star for the Athletics in the early 1900s. According to Lamont Buchanan in The World Series and Highlights of Baseball, the Athletics fans were fond of chanting, "If Eddie Plank doesn't make you lose / We have Waddell and Bender all ready to use!" Plank holds the franchise record for career victories, with 284.

In 1909, the Athletics moved into the major leagues' first concrete-and-steel ballpark, Shibe Park. This remains the second and last time in franchise history where a new ballpark was built specifically for the Athletics. In 1912, Mack bought the 25% of the team's stock owned by Jones and Hough to become a full partner with Shibe. Shibe ceded Mack full control over the baseball side while retaining control over the business side. However, Mack had already enjoyed a nearly free hand in baseball matters since the franchise's inception.