Vasundhara Raje, 60, who served as the chief minister in an earlier stint from 2003 to 2008, was administered the oath by Governor Margaret Alva at the Rajasthan assembly building here in the presence of thousands of BJP activists and leaders.
Senior BJP leader L.K. Advani, party president Rajnath Singh, BJP's prime ministerial candidate Narendra Modi, Chhattisgarh Chief Minister Raman Singh and his Madhya Pradesh counterpart Shivraj Singh, and former party president Nitin Gadkari, were among those who attended.
Former chief minister Ashok Gehlot of the Congress was also present.
A huge crowd of party workers thronged the Janpath Road - leading to the state assembly building - where sitting arrangement was made so that the people could watch the ceremony.
Over 2,000 policemen were deployed and traffic police put several road blocks and restrictions on movements, which caused inconvenience to people.
The BJP Dec 8 recorded a thumping victory in Rajasthan, ending five years of the Ashok Gehlot-led Congress rule.
BJP candidates were declared elected in 162 of Rajasthan's 199 constituencies where elections were held. In Churu constituency, polling was postponed till Dec 13 due to death of a Bahujan Samaj Party candidate.