Uttar Pradesh Governor Anandiben Patel administered the oath to Adityanath in a ceremony also attended by Union Home Minister Amit Shah, Defence Minister Rajnath Singh and BJP national president J P Nadda.
Adityanath was unanimously elected the BJP legislature party leader in UP on Thursday, after which he staked claim to form the government.
The swearing-in ceremony was held at Atal Bihari Vajpayee Ekana Stadium, the venue having the capacity to accommodate 50,000 people.
The BJP-led alliance had won 273 of 403 seats in the just-concluded Uttar Pradesh Assembly polls. While the BJP got 255 seats, its allies Nishad Party and the Apna Dal (S) bagged 18.