The St. Louis Post-Dispatch earlier reported the move Friday, citing multiple sources. The 37-year-old veteran is expected to join the Cardinals in Chicago on Saturday.
St. Louis had been using Tony Cruz and George Kottaras behind the plate since it lost Yadier Molina, who will miss at least two months as he recovers from July 11 surgery to repair a ruptured thumb ligament. The All-Star was hitting .287 with seven homers and 30 RBIs.
Kottaras was claimed off waivers from the Cleveland Indians two days after Molina's injury.
Pierzynski was designated for assignment by the Boston Red Sox on July 9. He was having the worst offensive season of his career, hitting just .254 with four home runs and 31 RBIs.