The former 60 Minute Makeover host will start working on the morning programme in the summer, appearing as a roving reporter.
Speaking to The People, Sweeney commented: "I'm thrilled. I won't be on the sofa but I can't wait to hit the road."
Sweeney's appointment follows the recent news that Adrian Chiles is to become one of the breakfast show's main anchors later this year.
The ex-Brookside actress added: "I'm so glad Adrian is joining. He's like a young Eamonn Holmes. Viewers will love him."
GMTV has been undergoing a revamp since January after ITV took full control of the slot.