Sylvester Urban "Blix" Donnelly (January 21, 1914 – June 20, 1976) was an American professional baseball pitcher.

A right-hander, Donnelly appeared in 190 Major League Baseball (MLB) games between 1944 and 1951 for the St. Louis Cardinals, Philadelphia Phillies and Boston Braves.

A lifelong resident of Olivia, Minnesota, Donnelly spent nine seasons in minor league baseball; in 1941, he had 28 wins and 304 strikeouts for the Class C Springfield Cardinals of the Western Association.

He was a member of the 1950 "Whiz Kids" edition that won the NL pennant; however, at age 36, Donnelly was one of the older players on the squad and did not appear in the 1950 World Series.

Donnelly's MLB career stat line includes 659 hits allowed in 6912⁄3 innings pitched, with 306 bases on balls, and 296 strikeouts.