Keith Watson stood for Greetland and Stainland ward under the Liberal Democrat banner.
But yesterday he told the party he was resigning and will be joining the Conservatives, who control the council.
Liberal Democrat group leader Janet Battye said: "This is shameful, within a week of having been re-elected as a Liberal Democrat councillor.
"We feel it is disloyal to his ward residents, who thought they were electing a person under one banner and actually find he is something else.
"We have asked him to do the honourable thing and resign his seat and stand in his new colours for re-election, but he has refused."
Conservative group leader Ann McAllister said Coun Watson was in the process of applying to the party and would be sitting on the Conservative bench next week.
"I am delighted to have an able and excellent ward member joining our party," she said.
"This is a decision he has made after long consideration and my understanding is that he does not feel that the Liberal Democrats are serving the people of the lower valley that he was elected to represent."
She added: "I know he will be an asset within the Conservative group. He knows what the people want and he feels that they would be better served under a Conservative administration."
Councillor Watson was unavailable for comment.