According to Deadline, the 42-year-old will play a love interest for Alan (Jon Cryer) named Gretchen.
The character is also the sister to Alan's friend Larry (DB Sweeney).
Williams-Paisley's recruitment reunites her with past According to Jim co-star Courtney Thorne-Smith.
Carl Reiner recently guest starred in an episode of Two and a Half Men, reprising his role as Evelyn's (Holland Taylor) wealthy suitor Marty Pepper.
CBS airs the show on Thursdays at 9.30pm ET.