McDonald had a successful first full season in Pittsburgh last year with a record of 9-9 and a 4.21 earned run average in 31 starts. He was acquired from the Dodgers during the 2010 season.
McCutchen led the Pirates with career highs of 23 homers and 89 runs batted in during the 2011 campaign, earning his first All-Star Game nod. He is a career .276 hitter in three big league seasons, all with Pittsburgh.
Walker, a Pittsburgh native, led the Pirates with a .273 batting average last season and is a career .280 hitter in his three seasons.
Other players agreeing to terms for the season were infielders Chase d'Arnaud, Matt Hague, Josh Harrison, Jordy Mercer, Yamaico Navarro and Gustavo Nunez; catcher Michael McKenry; outfielders Gorkys Hernandez, Starling Marte and Alex Presley; and pitchers Jared Hughes, Chris Leroux, Brad Lincoln, Jeff Locke, Daniel McCutchen, Kyle McPherson, Bryan Morris, Daniel Moskos, Rudy Owens, Tony Watson, Duke Welker and Justin Wilson.