William Ellsworth Blair (September 17, 1863 in Pittsburgh, Pennsylvania – February 22, 1890 in Pittsburgh, Pennsylvania) was a Major League Baseball player. A left-handed pitcher who batted from the left side, Blair had a listed playing weight of 172 pounds.

In an eight-year career as a professional, Blair spent one season in the major leagues. He started four games for the Philadelphia Athletics of the American Association in 1888, winning one and losing three. His 2.61 ERA was better than the league average, and he was also a positive contributor with the bat, posting a .308 batting average, .357 on-base percentage, and .385 slugging percentage in 14 plate appearances.

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. His obituary listed the cause of death as "influenza, which turned into pneumonia and typhoid fever". He was only 26 years old.