TV Guide reports that the actress will play Barb Tucker, the mother of Cameron (Eric Stonestreet), on the ABC sitcom.
She is said to have won the role ahead of Delta Burke, Dianne Wiest and Kathy Bates.
The role of Cameron's father, who was originally intended to appear in the episode, has since been written out.
Weston played Adele Delfino, the mother of Mike (James Denton), in a 2008 episode of Desperate Housewives and has recently starred in TNT cop show Memphis Beat.
Modern Family returns to ABC on September 22.