Sonia is in Russia as a personal guest of President Vladimir Putin.
The visit is considered to be proof of the strengthening strategic partnership between the two countries.
Sonia met her host on Wednesday.
During her talks with Putin in his hometown St. Petersburg, Sonia was accompanied by External Affairs Minister K Natwar Singh and Indian Ambassador to Russia Kanwal Sibal.
This is Sonia's fourth visit to Russia and the first after becoming UPA chairperson.
Earlier, she had visited Soviet Russia with her husband the late Rajiv Gandhi, the then prime minister.
Photograph: AP Photo/Dmitry Lovetsky