The 64-year-old has been sworn in as the state's 22nd Chief Minister by Governor H R Bhardwaj. Mr Siddaramaiah, a stated agnostic, took oath in the name of truth.
The venue was shifted to a stadium to accommodate the thousands of people expected; they did not disappoint. About 50,000 people had gathered from Mr Siddaramaiah's Mysore district and nearby areas alone.
Many Congress leaders too attended the ceremony, among them Union Minister Veerappa Moily and Maharasthra Chief Minister Prithviraj Chavan.
The Congress had last come to power in Karnataka in 2004 as part of a coalition government with the Janata Dal (Secular), a partnership that did not last long. This time it has been handed a clear majority, winning 121 seats in the 224-member Assembly.
Mr Siddaramaiah was chosen the Congress Legislature Party leader on Friday, edging out party veteran and Union Labour Minister M Mallikarjuna Kharge. He has been Deputy Chief Minister twice before and has presented seven Budgets in the state.
He is relatively new in the Congress. An HD Deve Gowda protege, Mr Siddaramaiah left the Janata Dal (Secular) six years ago and walked over to the Congress. A lawyer-turned-politician, Mr Siddaramaiah had narrowly missed the Chief Minister's seat in 1996 when Mr Deve Gowda quit to become Prime Minister.
He is from the Kuruba community, the third largest in the state.
The new Chief Minister, along with state Congress chief G Parameshwara, is expected to visit Delhi tomorrow to meet the Congress high command to discuss his new Cabinet.