Party General Secretary Janardhan Dwivedi, who announced her return, said Gandhi is fine, the NDTV television channel reported. The report quoted sources as saying she would not meet anyone soon.
Some news reports have said she was in the United States but that has not officially been confirmed.
The Italian-born 64-year-old Gandhi, whose party leads the coalition government, was away for a month and her return to New Delhi comes a day after an explosion at an Indian high court in the capital killed and injured dozens of people.
She is regarded as the most powerful politician in the world's largest democracy although she holds no post in the government of Prime Minister Manmohan Singh.
Her absence from New Delhi was strongly felt as the government, already burdened by high inflation and other economic problems, faced a tough situation last month in handling the hungerstrike of Anna Hazare, who is leading a campaign against corruption in the country. Singh's government has been hit by a number of corruption and graft scandals although he personally has not been affected by them.
Sonia Gandhi is the widow of former Prime Minister Rajiv Gandhi, who was assassinated in May 1991.