Rosenthal's successful rookie season carried over into the postseason, making him instrumental in the Cardinals' playoff run as they needed bullpen help. In his playoff debut against the Washington Nationals in the National League Division Series (NLDS), he struck out the side, retiring Jayson Werth, Bryce Harper, and Adam LaRoche while allowing a single to Ryan Zimmerman. Six more scoreless appearances followed as the Cardinals advanced to the National League Championship Series (NLCS). In all, Rosenthal struck out 15 of total 30 batters faced in the 2012 playoffs.