Sliema councillor Michael Briguglio, who had been elected to the council on Alternattiva Demokratika's ticket, has crossed over to the Nationalist Party.
A former AD chairman, Dr Briguglio became a member of the Nationalist Party last May.
In a letter to PN general secretary Rosette Thake, Dr Briguglio requested membership in the PN's College of Councillors, joining the party's councillors in Sliema.
The PN accepted Dr Briguglio's request.
After resigning from AD last June, the Green Party had asked Dr Briguglio to also step down as a member of Sliema council.
Dr Briguglio had told AD he would keep representing his constituents who had been electing him since 2003.