Digambar Kamat, the No 2 man in the previous Manohar Parrikar government in Goa, quit his assembly seat on Thursday.
He handed over his resignation to legislature secretary Sudhir Narvekar.
This has cleared the way for Congress leader and Chief Minister Pratapsing Rane to win the confidence vote on February 28.
With Kamat's resignation, the BJP now has 17 members in the assembly, while the ruling Congress-led alliance remains at 18.
As Speaker Vishwas Satarkar belongs to the BJP, the House of 35 needs 18 to have majority.
Governor S C Jamir had installed Rane as CM on February 2, after the Parrikar government was dismissed amidst controversy.
Two alliance partners had withdrawn their support to the BJP-led government. Four BJP legislators had also quit their seats to reduce the saffron strength in the 40-member House.