Party sources said that she left for abroad on Wednesday night for a "routine medical check-up" and will return only after Holi on Monday.
Though they did not let in on where Gandhi has gone for the check-up, there was speculation that she was in the United States where she had undergone treatment for an undisclosed ailment.
Gandhi, who has not been keeping well for quite some time, held a closed-door meeting with senior party leaders and her deputy Rahul Gandhi before her departure. Rahul, Congress vice president, will discharge the functions of the party president in her absence.
Rahul, tipped to succeed her over time, spearheaded Congress's election campaign in the five states of Uttar Pradesh, Uttarakhand, Punjab, Goa and Manipur.
Sonia Gandhi has been out of action much of the time after having taken ill during a roadshow in Varanasi, Prime Minister Narendra Modi's constituency, on August 2.
She was flown to the national capital and admitted to the Army's Research and Referral Hospital before being shifted to Sir Ganga Ram Hospital. Gandhi was again admitted to Sir Ganga Ram Hospital in November for viral fever and discharged after a few days.