Cllr Jean Ashworth, who represents Smallbridge and Firgrove Ward on Rochdale Council, has resigned from the Lib Dems and joined the Conservatives.
"I feel it is going to be many years before the Liberal Democrats recover in Rochdale and our town cannot wait that long.
"We need to see improvements in our town as soon as possible and we need effective leadership that will work together for their benefit of everyone.
"I feel that the leader of the Rochdale Conservatives and his team put the borough before political differences."
The Conservatives are now the main opposition to the Labour-run Council.