The councillors switched their allegiance at the BJP's Delhi headquarters in the presence of party leader Mukul Roy and Darjeeling MP Raju Bista.
Roy said the 32-seat municipality now had an effective strength of 30 after the death of a councillor and the resignation tendered by another.
"With 17 GJM councillors crossing over, BJP now has a majority in Darjeeling municipality," said Roy.
He said the councillors have submitted a letter to the Darjeeling district magistrate announcing their change in loyalty.
Darjeeling MP Raju Bista said this was only the beginning, and the BJP would soon take control of Kalimpong, Kurseong and Mirik municipalities with the councillors of these civic bodies also crossing over to its fold.
He demanded that the Gorkha Territorial Administration (GTA) now being run by the Tamang-Anit Thapa faction of the GJM in alliance with the Trinamool should be dissolved and an audit carried out in view of the "corruption" in the body.
"The voice of the people is with BJP and the GJM's original group, as also its allies like GNLF. So, on ethical grounds, GTA should be dissolved, because those running the GTA have not been elected by the people," Bista said.
Late last month, the BJP came up with an impressive show in Bengal, winning 18 of the 42 Lok Sabha seats, up from two constituencies it won in 2014. Soon after, Trinamool councillors joined the saffron party in bulk in four municipalities - Bhatpara, Naihati, Halishahar and Kanchrapara - enabling the BJP to gain majority in the civic bodies.
( With inputs from IANS )