Sonowal (50), who joined BJP less than two years ago, will take over from Ranjit Dutta later in the month.
"Our aim is to protect the rights of genuine Indian citizens and the indigenous people of Assam. The 11-year-long reign of Congress in Assam has ruined the state," Sonowal said from Dibrugarh after his name was declared.
Sonowal's name as the new state BJP chief was announced by the party's national General Secretary Vijay Goel in New Delhi.
"We hope to successfully win future electoral battles by taking the people of all sections along with us... The Congress government has no policy," Sonowal said in response to a question regarding BJP plans for the coming panchayat polls in Assam and the 2014 Lok Sabha elections.
Sonowal was the president of All Assam Students' Union (AASU) from 1992 to 1999. In 1999, he joined the Asom Gana Parishad (AGP).
He was elected to the state Assembly from Moran constituency in 2001.
In 2004, he was elected for Lok Sabha from the Dibrugarh seat.
Before Sonowal's victory, Congress had never lost the Dibrugarh seat in Lok Sabha polls.
The leader, however, lost the 2009 Lok Sabha polls at the hands of Paban Singh Ghatowar, who is now a Union minister.
In February 2011, Sonowal joined BJP along with some other AGP workers. He was later inducted as a member of BJP's national executive.