Louis Laddie Koupal (December 19, 1898 – December 8, 1961) born in Tabor, South Dakota, was a pitcher for the Pittsburgh Pirates (1925–26), Brooklyn Robins (1928–29), Philadelphia Phillies (1929–30) and St. Louis Browns (1937).

Koupal was of Czech descent. As a rookie in 1925, he appeared in six games, all in relief, for the Pirates, who would go on to win the World Series, though Koupal did not pitch in the postseason.

In six seasons, Koupal had a 10–21 won-lost record, 101 appearances, with 35 starts, 12 complete games, 40 games finished, 7 saves, 335+1⁄3 innings pitched, 436 hits allowed, 255 runs allowed, 208 earned runs allowed, 23 home runs allowed, 156 walks allowed, 87 strikeouts, 5 hit batsmen, 11 wild pitches, 1,560 batters faced and a career ERA of 5.58.

Koupal died in San Gabriel, California at the age of 62.