Following are the top developments in this story:
Mr Yediyurappa, wearing a green silk stole over his white shirt, took oath alone in the third oath ceremony since last year's Karnataka polls. The Congress, which lost power on Tuesday, boycotted what its leaders called an "unholy event".
"The difference between the BJP government and the coalition government is that we work for the development... Forget and forgive is our mantra," Mr Yediyurappa told the media after taking oath.
Before his swearing-in, the 76-year-old changed the spelling of his name from "Yeddyurappa" to "Yediyurappa". The three-time chief minister, who has never completed a term, wants to take no chances this time. His first term lasted one week and the second term, three years. The third stint was just 48 hours long.
This morning, Mr Yediyurappa surprised many by heading to Governor Vajubhai Vala's home and asking to be sworn in, after a two-day wait for a go-ahead from his leadership in Delhi. The BJP was believed to be playing it safe to secure its numbers in the assembly but Mr Yediyurappa reportedly persuaded BJP chief Amit Shah to let him take charge in Karnataka.
Sources say the clearance came after Karnataka BJP leaders assured Delhi that they had the support of the rebels and would win the vote. The rebels reportedly guaranteed their vote for the BJP in a video call with Karnataka leaders.
After the Karnataka election in May last year that yielded no clear winner, Mr Yediyurappa took charge as chief minister but resigned two days later, just before a trust vote, knowing he didn't have the numbers. The Congress and Mr Kumaraswamy's Janata Dal Secular then tied up to keep the BJP out of power.
The BJP moved swiftly a day after Karnataka Speaker KR Ramesh Kumar disqualified three of the 17 rebel lawmakers whose exits brought down the Congress-JDS coalition. In a floor test on Tuesday after four days of prolonged debate that the BJP alleged were an attempt to stall the vote, the coalition could manage only 99 votes to the BJP's 105.
The disqualifications bring down the strength of the House to 222, and the majority mark drops to 112. The BJP has 105 lawmakers and the support of an independent; another independent legislator was disqualified. The Congress-JDS has 100 (including a member who stayed away from the trust vote claiming he was unwell).
Fifteen legislators set the Congress-JDS coalition's downfall in motion with mass resignations earlier this month. They promptly flew out of Bengaluru to Mumbai, where they have been staying in a five-star hotel. The coalition accused Mr Yeddyurappa of unleashing "Operation Lotus", the name given by critics to what they call the BJP's strategy of seizing power by drawing away lawmakers with money or other inducements.
Congress and JDS leaders skipped the oath. Describing Mr Yediyurappa as a "corruption icon and former Jail Bird" in a tweet, the Karnataka Congress accused him of using "his excellent Horse Trading skills to subvert democracy and come to power". The party said "the people of Karnataka remember his disastrous tenure as CM between 2008-2011, which ended with BSY (Yediyurappa) in Jail. History is all set to repeat again."