Frank Bissell "Lefty" Killen (November 30, 1870 – December 3, 1939) was a professional baseball player.

He was a left-handed pitcher over parts of ten seasons (1891–1900) with the Milwaukee Brewers, Washington Senators (NL), Pittsburgh Pirates, Boston Beaneaters and Chicago Orphans.

[4] In 1893, Killen posted a league-best 36 victories against fourteen defeats.

For his career, he compiled a 164–131 record in 321 appearances, with a 3.78 ERA and 725 strikeouts.

As a hitter, Kileen posted a career .241 batting average (241-for-998) with eleven home runs, 127 RBI, 151 runs scored and drew 131 bases on balls.