William Pleis III (born August 5, 1937) is a retired American professional baseball player, a left-handed pitcher who appeared in 190 Major League games between 1961 and 1966 for the Minnesota Twins.

[1] Pleis, listed at 5 feet 10 inches (1.78 m) tall and 170 pounds (77 kg), was a relief pitcher for all but ten of his 190 big-league appearances.

In 1965, he equaled his career season-high for saves (four) and won four other games as the Twins captured their first American League pennant and the franchise's first since 1933, when it was located in Washington.

He appeared in the 1965 World Series against the Los Angeles Dodgers, a team he would serve as a longtime scout after the end of his playing career.

He allowed 269 hits and 127 bases on balls in 280+2⁄3 innings pitched in the Majors, with 184 strikeouts, 13 saves and one complete game.