Councillor Adair said the move would not affect her constituents
A fourth councillor has defected to the Liberal Democrats boosting the party's majority on Portsmouth City Council.
Tory councillor Margaret Adair followed "good friend" and fellow Conservative member Lee Hunt across the floor.
Councillor Hunt, alongside independents Leo Madden and Jason Fazackarley, switched to the Lib Dems last Tuesday, giving the party overall control.
Conservative councillor Alistair Thompson said he was "disappointed" but was not expecting any more defections.
Councillor Adair, member for Central Southsea, said: "I am supporting councillor Hunt who is a dear colleague.
"But I also wanted to do it for myself.
"I have sometimes thought I get on better with them than the party I was in.
"My constituents voted for me as a person and this move will not affect that."
The decision means the Lib Dems now have 23 councillors to the Tories' 17 and two for Labour.
The defections last week gave the party overall control of Portsmouth for the first time since 2000.
Councillor Thompson said: "All of us know Margaret and Lee are very good friends, it is not a surprise.
"I am not concerned as a party, we know that across Portsmouth we have had some very good support.
"It is disappointing for Portsmouth South, but I think it is probably the last one [defection]."