He was administered the oath of office and secrecy by Governor Ganeshi Lal in the presence of thousands of people and dignitaries at Idco Exhibition Ground here.
The 72-year-old BJD chief has become one of the longest-serving chief ministers of the coastal state.
Twenty ministers including 11 cabinet ministers and nine ministers of state were also administered the oath of office and secrecy.
The cabinet ministers are Ranendra Pratap Swain, Bikram Keshari Arukh, Prafulla Kumar Mallick, Niranjan Pujari, Padmanabh Behera, Pratap Jena, Arun Kumar Sahoo, Sudam Marndi, Susant Singh, Naba Kishore Das and Tukuni Sahu.
The ministers of state are Ashok Chandra Panda, Samir Ranjan Dash, Jyoti Prakash Panigrahi, Dibya Shankar Mishra, Premananda Nayak, Raghunandan Das, Padmini Dian, Tusharkanti Behera and Jagannath Saraka.
For the first time in the last two decades, Patnaik held the swearing-in ceremony in open. It was attended by some prominent dignitaries from different fields including industry, art, culture, education and civil society.
Patnaik's elder sister and noted writer Gita Mehta and his brother Prem Patnaik were also seen in the event.
The Biju Janata Dal, which secured 112 seats in the 147-member Odisha Assembly in the recently-concluded election, held along with Lok Sabha polls, has been in power in the state since 2000.
( With inputs from IANS )