John P. Cassidy (1855 – July 3, 1891) was an American outfielder in Major League Baseball from 1875 to 1885. He was a pitcher in his first season, 1875, with the Brooklyn Atlantics but went 1–20 that season and was moved to the outfield the following year.

Cassidy died at the age of 36 in his hometown of Brooklyn, New York of dropsy, and is interred at Flatbush Cemetery.

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