John Patrick Parnell "Patsy" Cahill (April 4, 1865 – October 31, 1901) was a Major League Baseball outfielder. In addition to playing the outfield, Cahill also played third base, shortstop and he also pitched 10 games.

There is speculation that Cahill was the inspiration for Ernest Lawrence Thayer's poem "Casey at the Bat" after Thayer reportedly saw Cahill play in .

Cahill signed as a free agent with the Philadelphia Quakers in December 1883 but on May 19, 1884 his contact was purchased by the Columbus Buckeyes from the Quakers. Cahill began his professional career with the Buckeyes of the American Association in 1886. He played in 59 games and batted .219 with 46 hits, three doubles, three triples and six base on balls in 210 at bats.

In 1885 Cahill played for the Atlanta ball club of the Southern Association. He returned to the major leagues in 1886 playing for the St. Louis Maroons of the National League. Cahill hit only .199 with 92 hits with 17 doubles, six triples, one home run, 32 RBIs with 16 stolen bases.