February 27 – The sacrifice fly rule is adopted. No time at bat is charged if a run scores after the catch of a fly ball. The rule will be repealed in 1931, then reinstated (or changed) several times before gaining permanent acceptance in 1954.
September 23 – The Chicago Cubs and New York Giants, involved in a tight pennant race, (also involving the Pittsburgh Pirates) were tied in the bottom of the ninth inning at the Polo Grounds. The Giants had runners on first and third and two outs when Al Bridwell hit a single to center field, scoring Moose McCormick from third with the Giants’ apparent winning run, but the runner on first base, rookie Fred Merkle, thinking the game was over, went half way to second and then sprinted to the clubhouse after McCormick touched home plate. As fans swarmed the field, Cub infielder Johnny Evers retrieved the ball and touched second. A forceout was called at second base, nullifying the single, and since there were 2 outs when the play started, the run was also nullified and the inning ended. The game was declared a tie and would be made up at the end of the season if the Cubs and Giants were tied for first place. The incident became known as "Merkle's Boner".
September 26 – Ed Reulbach of the Chicago Cubs pitches two shutouts in the same day, whitewashing the Brooklyn Superbas 5–0 on a five-hitter and 3–0 on a three-hitter. The entire doubleheader is played in less than three hours. Reulbach allows five hits in the a.m. game, and is even stingier in the afternoon, yielding three hits and a walk. Kaiser Wilhelm and Jim Pastorius are the losing pitchers.
In a game involving the Cleveland Naps and the Chicago White Sox, Ed Walsh struck out 15 Naps and walked one batter, pitching a complete game, but it was not enough as Addie Joss pitched a perfect game, as the Naps beat the White Sox, 1-0, during the heat of a pennant race. Cleveland center fielder Joe Birmingham scored the game's only run. Joss' perfect game is the second in the modern era and fourth all time. It is perhaps the finest pitching duel in baseball history.
October 14 – The baseball season of 1908 ends with the lowest runs per game average in major league baseball history at 3.38. This outcome was likely an important factor in the decision to trial a livelier "cork center" ball in both leagues the following season.
January 14 – Sim Bullas, 45, outfielder for the 1884 Toledo Blue Stockings.
January 14 – Henry Krug, 41, utility for the 1902 Philadelphia Phillies.
February 20 – Wallace Terry, 57, first baseman/outfielder for the 1875 Washington Nationals.
March 12 – Fred Ketcham, 32, outfielder for the Louisville Colonels (1899) and Philadelphia Athletics (1901).
March 27 – Forrest Crawford, 26, shortstop who played 1926 through 1927 or the St. Louis Cardinals.
March 30 – Charlie Sweasy, 60, second baseman for 1869 Cincinnati Red Stockings. Played seven years in the National Association and the National League.
April 6 – Jim Brown, 47, pitcher and outfielder for two seasons, 1884 and 1886.
April 10 – Mike Griffin, 43, center fielder for Baltimore and Brooklyn who batted .300 six times, scored 100 runs ten times; led league in runs and doubles once each.
April 13 – John Kelly, 49, 19th century catcher, manager and umpire.
April 20 – Henry Chadwick, 83, the "Father of Baseball", who through his writings, analysis of statistics and service in developing the sport's rules played a principal role in establishing baseball as the "national pastime"; devised the box score, developed scoring system which enabled recording of every play, authored the first rule book in 1858, and created statistics including batting average and earned run average; worked to revise sport's rules so as to balance offense and defense, and to increase mental demands as well as physical ones.
September 7 – Bill Morgan, 52, outfielder and shortstop who played with the Pittsburg Alleghenys (1883) and Washington Nationals (1884).
September 14 – Ike Van Zandt, 32, outfielder and pitcher who played for the New York Giants (1901), Chicago Cubs (1904) and St. Louis Browns (1905).
September 18 – Dickey Pearce, 72, shortstop (in the sport's earliest era) whose career spanned the years 1856 to 1877; introduced the bunt and pioneered defensive play at his position, later became an umpire.
September 28 – Tom Pratt, 64, played at first base for one game with the 1871 Philadelphia Athletics.
November 5 – Pat Hannivan, 42, outfielder and second baseman for the 1897 Brooklyn Bridegrooms.
December 8 – Frank Griffith, 36, pitcher for the Chicago Cubs (1892) and Cleveland Spiders (1894).
December 10 – Wild Bill Widner, 41, pitcher who posted a 22-36 record and a 4.36 ERA with the Red Stockings, Nationals, Solons and Kelly's Killers from 1887 to 1891.
December 19 – Reddy Foster, 44, pinch hitter for the 1896 New York Giants.
December 26 – Charlie Householder, 52, third baseman/left fielder/shortstop who hit .239 in 83 games for the 1884 Chicago/Pittsburgh team of the Union Association.
December 26 – Shadow Pyle, 47, pitcher for the Philadelphia Quakers (1884) and Chicago White Stockings (1887).