The development could be a setback for Congress as Radadia, who belongs to Leuva Patel community, is a prominent leader of farmers and cooperative sector from Saurashtra.
Several cooperative movement leaders from Saurashtra too joined BJP at the function attended by thousands of people.
Modi offered a saffron scarf to Radaida, formally inducting him into BJP.
"It is for the first time in the history of BJP that such a large group of people is joining the party," Modi said adding he welcomed Radadia with an "open heart". Modi also described Radadia as a "fighter and a mass leader".
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"Congress people are saying they won't be affected by my joining BJP. They will see the effect in 2014 elections".
Radadia had last October brandished a gun at toll booth at Karzan and his action was caught on CCTV. He was Porbandar MP then.
He and his son Jayesh fought and won state assembly elections from Dhoraji and Jetpur respectively in December, after which Radadia resiged as MP.