Rupani, 61, and his deputy Nitin Patel were administered the oath of office and secrecy by Governor O P Kohli at a grand ceremony in Gandhinagar.
Prime Minister Narendra Modi, BJP president Amit Shah and chief ministers of various BJP-ruled states attended the ceremony.
Before the swearing-in ceremony, Rupani, sporting an orange-coloured jacket, and Patel greeted the chief ministers of BJP-ruled states who came to attend the ceremony.
Rupani and his wife also offered prayers at the Panchdev Mahadev temple here before the swearing-in ceremony.
The opposition Congress, which had won 61 seats in 2012, managed to increase its tally to 77. The strength of the Congress and its allies in the new House is 80.
Rupani and Nitin Patel were elected as the leader and deputy leader, respectively, of the BJP legislature party at its meeting on December 22.
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