Muthalik later told reporters that he joined BJP with the objective of ensuring that Narendra Modi becomes the country's next prime minister.
He has had a love-hate relation with BJP, which was evident when the saffron party had accused him of misusing Modi's name for raising funds.
He joined RSS in 1975 and Shiv Sena in 2005, but left the latter in 2006. This is the first time he is officially being associated with the party.
BJP and Sangh Parivar leaders feel Muthalik's entry into the party can tilt the scales, especially in Belgaum, as people divided on the basis of mother tongues Kannada and Marathi, will come together under the Hindutva plank.
BJP leaders also feel his entry will benefit the party in other parts of the district and neighbouring districts.
Muthalik shot into the limelight when Ram Sene workers allegedly attacked girls in a pub in Mangalore in 2009.
The hardcore Hindutva leader is facing several cases, including one relating to promoting enmity between two communities.