64-year-old Siddaramaiah was administered the oath of office and secrecy by Governor HR Bhardwaj in front of thousands of his supporters at the Kanteerava Stadium.
Sporting white colour silk dhoti and shirt, Siddaramaiah was sworn alone and he took the oath in the name of 'Truth'.
Siddaramaiah is expected to constitute the full-fledged ministry after consultations with the central leadership some time this week, a difficult task on hand considering the number of ministerial aspirants.
Union Ministers Mallikarjun Kharge, whom Siddaramaiah had piped in the race for chief ministership and M Veerappa Moily besides Maharashtra Chief Minister Prithiviraj Chavan, Congress General Secretaries Madhusudan Mistry, in charge of State party affairs, Oscar Fernandes and BK Hariprasad were present.
The swearing in ceremony is usually held on the portals of Vidhana Soudha, the state secretariat, or at the Raj Bhavan.
Vidhana Soudha was avoided as Bangalore Metro works are on in front of the building. Siddaramaiah was chosen the Congress Legislature Party leader on Saturday after AICC observers headed by Defence Minister A K Antony asked party legislators to express their choice in writting.
Coming from the "Janata Pariwar" stable, Siddaramaiah had led the Congress battle against the BJP along with KPCC Chief G Parameshwara, paving the way for the return of Congress to power inflicting a humiliating defeat on the ruling party and winning 121 seats in the 224-member Assembly in the May 8 polls.