Chief Justice of Andhra Pradesh High Court A R Dave administered oath of office and secrecy to Rosaiah at the Raj Bhavan.
The 76-year-old will serve as the caretaker chief minister till the Congress legislature party elects a new leader.
Speaking to reporters Rosaiah said, "Usually one feels happy while taking oath but I have taken oath as the caretaker chief minister with a heavy heart. Since this is a constitutional requirement, I agreed to serve as caretaker chief minister till a new leader is elected."
Rosaiah was sworn in caretaker chief minister following the death of Chief Minister YS Rajasekhara Reddy in helicopter crash on Wednesday, Sep 2.