The oath ceremony was taking place with the presence of numerous eminent guests, which included LK Advani, Narendra Modi and Chief Ministers from various other states.
The BJP, which did not field any Muslim candidate in UP, has appointed former cricketer Mohsin Raza as a minister of state.
Keshav Prasad Maurya, the party's state unit chief, and Dinesh Sharma, BJP's national vice president, were inducted as deputy chief ministers.
"At the end of the day, this is a party that is driven by orders they receive from the Rashtriya Swayamsevak Sangh headquarters in Nagpur... we all know about the very inflammatory language used by Yogi Adityanath over the last several years", he said.
Analysts also see Mr Adityanath's appointment as a sign that the BJP would be pushing its Hindu nationalist agenda with greater fervour in the run-up to the 2019 general election, and as a test of Mr Modi's pledge of inclusive governance.
A galaxy of VIPs attended the swearing-in at the Kanshi Ram Smriti Upvan.
"If the name of the new chief minister is revealed before the meeting of the newly-elected MLAs, then the relevance and sanctity of the meeting would be lost", he said.
Narendra Modi looks on as Yogi Adityanath (extreme right) takes oath as Uttar Pradesh chief minister in Lucknow on Sunday. The selection of Yogi as Chief Minister has started tongue waging that BJP was out to implement its Hindutava agenda by marginalizing members of minority community. The Prime Minister had already left for Dehradun to witness the swearing of Uttarakhand chief minister Trivendra Singh Rawat.
"We want to assure the people of UP that our government will take the state on the path of development and prosperity and would do everything that it takes to do so", Yodi Adityanath said.
Known for his inflammatory speeches against Muslims, Mr Adityanath lauded Mr Trump for banning refugees and immigrants from several Muslim-majority countries from entering the U.S., and said India needed similar action to check terrorism.
In the recently-concluded Uttar Pradesh assembly elections, the BJP secured 312 seats, while its allies Apna Dal (Soneylal) and Suheldev Bhartiya Samaj Party got nine and four seats respectively. After the landslide victory in Uttar Pradesh, there were questions within the party about whether a hardliner should be made the chief minister. Party insiders said the overwhelming mandate has posed a fresh challenge for the BJP to meet sky-high expectations.