William Ellsworth Blair (September 17, 1863 – February 22, 1890) was an American professional baseball player.

A left-handed pitcher who batted from the left side, Blair had a listed playing weight of 172 pounds (78 kg).

He started four games for the Philadelphia Athletics of the American Association in 1888, winning one and losing three.

Blair had signed a contract to spend the 1890 season with the Chicago Cubs, but he died as a result of illness before games began, during the first outbreak of the 1889–1890 pandemic.

His obituary listed the cause of death as "influenza, which turned into pneumonia and typhoid fever".