Two former Uttar Pradesh ministers, three ex-MLAs and several leaders from other parties joined the BJP in Lucknow on Sunday.
UP BJP chief Swatantra Dev Singh inducted them into the party and said their joining will strengthen the BJP.
Among the former ministers who joined the party was Jainarayan Tiwari from Sultanpur, according to a statement.
A three-time minister, he has been associated with the Congress and the Samajwadi Party in the past.
Mr Tiwari was close to former UP chief minister Sripati Mishra.
In an apparent reference to the Congress, Prime Minister Narendra Modi had recently alluded to the removal of Mishra from the chief minister's post in the 1980s.
Vijay Mishra, who was an SP MLA from Ghazipur and a former minister, also joined the BJP.
The former MLAs who joined the ruling party are Ram Shiromani Shukla (Pratapgarh), Madan Gautam (Auraiya) and Abhimanyu Pratap Singh (Ayodhya).
The others who joined the BJP are Manoj Divakar, Jagdev Kureel (Kanpur), Ashok Kumar Singh, Dharmendra Pandey (Unnao) and Akhilesh Verma (Lakhimpur Kheri).
Laxmikant Bajpai, the chairman of the party's joining committee, and Deputy Chief Minister Dinesh Sharma were also present.