Gellar will not reprise her role as Kendall Hart, though, which she played on the soap from 1993-95.
"I don't want to be Kendall. That's Alicia's role," Gellar said, alluding to current star Alicia Minshew. "I have no idea what I'm going to be doing."
Gellar will tape her one-day stint during a break from "Ringer," her new series on The CW, in which she plays twin sisters with a dark side.
"All My Children," which will air its final episode Sept. 23 after a 40-year run, was canceled last spring along with the long-running soap "One Life to Live."
"One Life to Live" will wrap up in January 2012, capping 42 years on the air.
"All My Children," set in a fictitious suburb of Philadelphia, kicked off the careers of many celebrities, including Josh Duhamel, Kelly Ripa and Mark Consuelos.