The Pioneer League is a minor league baseball league which currently operates in the Rocky Mountain region of the United States. In the past, it also operated in adjoining portions of Canada. It is classified as a Rookie league, and is almost exclusively the first professional league in which many players compete; most of the players have just been signed out of high school. The Pioneer League is a short-season league operating from June to early September.
Along with the Appalachian League, it forms the second-lowest rung on the minor league ladder. Although classified as a Rookie league, the level of play is slightly higher than that of the two "complex" Rookie leagues, the Gulf Coast League and Arizona League. Unlike these two leagues, Pioneer League games charge admission and sell concessions.
The Pioneer League began in 1939 with six teams in Idaho and Utah. With players in short supply due to World War II, the league suspended operations for the 1943 through 1945 seasons. In 1948, the league expanded into Montana. At that time, several of the teams were operating as minor league affiliates of Pacific Coast League teams, which were unsuccessfully trying to grow into a major league. When the Los Angeles Dodgers displaced the Hollywood Stars PCL team, they moved to Salt Lake City, Utah, taking away the Pioneer League's largest market. By 1959, the league was down to six teams, and by 1964, there were only four. By the end of the 1970s there were eight teams, a number that remains to this day and is not likely to change without further expansion or contraction within Major League Baseball.