Billy Wagner, American baseball player and coach
William Edward Wagner (born July 25, 1971), nicknamed "Billy the Kid", is an American former professional baseball pitcher who played 16 seasons in Major League Baseball (MLB). He pitched for the Houston Astros (1995–2003), Philadelphia Phillies (2004–2005), New York Mets (2006–2009), Boston Red Sox (2009), and Atlanta Braves (2010). Wagner is one of only six major league relief pitchers to accumulate at least 400 career saves. A left-handed batter and thrower, Wagner stands 5 feet 10 inches (1.78 m) tall and weighs 180 pounds (82 kg).

A natural-born right-hander, Wagner learned to throw left-handed after fracturing his arm twice in his youth. His 11.9 strikeouts per 9 innings pitched ratio (K/9) is the highest of any major league pitcher with at least 800 innings pitched. He was a seven-time All-Star and the 1999 National League (NL) Rolaids Relief Man Award winner. He finished in the top ten in saves in the NL ten times, and in the top ten in games finished nine times.