Governor Vajubhai Vala administered the oath of office and secrecy to the Yediyurappa, 76, at a ceremony at the Raj Bhavan.
The Yediyurappa-led BJP ministry is assuming office three days after the collapse of the Congress-JD(S) government with the defeat of the motion of confidence moved by chief minister H D Kumaraswamy in the assembly by 99-105 votes.
Yediyurappa, who is president of the BJP's Karnataka unit, met the Governor on Friday morning and requested him to administer the oath of office and secrecy.
Before the swearing in, Yediyurappa said he would decide on members to be inducted into the ministry after consulting party president Amit Shah.
This is the fourth stint for Yediyurappa as the Chief Minister-- the last one was after the May 2018 Assembly polls, when he barely lasted three days after being sworn in.
The new chief minister reverted to the earlier English spelling of his name B S Yediyurappa, apparently influenced by numerology.