Dhananjay Singh, a gangster-turned-politician, held a meeting with his supporters on Tuesday evening before announcing his support for the BJP.
Dhananjay Singh wields considerable support in the constituency and is often referred to as the kingmaker.
The BJP candidate in Jaunpur is Kripa Shankar Singh, a former minister of state in the Maharashtra government.
The move comes a week after the BSP cancelled Dhananjay Singh's wife Shreekala Singh's ticket for the Jaunpur Lok Sabha seat.
Dhananjay Singh had said the move was a conspiracy to humiliate him.
"My wife is feeling hurt due to this," he said earlier.
The BSP has nominated its sitting MP Shyam Singh Yadav for the seat.
Dhananjay Singh, who was elected as an MP from Jaunpur in 2009 on a BSP ticket, said, “The BSP has betrayed me for the fourth time. In 2012, 2014, and 2017, the party assured me of a ticket but cheated me at the last moment."
It is noteworthy that earlier this week, the Lokayukta launched a probe against Dhananjay Singh over allegations of an illegal land deal, which is also being investigated by the Enforcement Directorate.
On March 6, a court in Jaunpur sentenced Dhananjay Singh to seven-year rigorous imprisonment in a 2020 case of kidnapping and extortion of a Namami Gange project manager.
However, the Allahabad High Court granted him bail on April 27 but refused his plea to suspend or stay the sentence.
The Jaunpur constituency goes to the polls on May 25.
In a related development, former minister Raghuraj Pratap Singh a.k.a. Raja Bhaiyya on Tuesday held a meeting with his supporters, before announcing that he would not support any party, and his supporters were free to make their choice.
Raja Bhaiya, who heads the Jansatta Dal, wields considerable influence in Kunda and Babaganj - two Assembly segments under the Kaushambhi Lok Sabha seat.
Vinod Sonkar, the BJP candidate in Kaushambhi, met Raja Bhaiya earlier in the day to seek his support.
Interestingly, both Dhananjay Singh and Raja Bhaiya held meetings with Union Home Minister Amit Shah in the past three days.