The Seinfeld star will play a teacher in the fifth episode of the show's sophomore season, TV Guide reports.
His character, Richard, is suspended after failing a student who described Darwinism as "a crock". Harriet (Kathy Bates) ends up taking the case to court.
Alexander is best known for his role as George Costanza in Seinfeld. He also appeared on Curb Your Enthusiasm and recently guest starred on Franklin & Bash.
In January, Alexander revealed that he was working on a new pilot for CBS.
Mark Valley, Jean Smart, Karen Olivo and Alfred Molina have all signed up to appear in the second season of Harry's Law, which will premiere on September 21 on NBC.