It means there will be two by-elections in Hastings this May, which could demolish the Tories' new-found majority.
Michael Lambrechs, popular across the political divide for his easy-going attitude and boundless enthusiasm, handed in his resignation on Wednesday.
For several months he has been living in Tarragona, Spain, apparently for health reasons.
He told the Hastings Observer: "We all have to make tough decisions from time to time and this one has been most difficult.
"I believe more decisions should be made locally, and therefore a councillor must be close to his constituents.
"I have always advocated that councillors should be residents in their wards, so they are part of their communities and always in touch.
"Sadly, my future plans no longer allow this."
Mr Lambrechs' resignation comes hot on the heels of Daniel Poulter's.
He quit last week because he will be pursuing a career in London.
If their seats - in Silverhill and Gensing wards - are taken by the opposition parties, the Conservatives will no longer have an overall majority.
It means Tory decisions could be blocked by a LibDem-Labour alliance.
Labour leader Jeremy Birch said: "After months living in Spain, Mike Lambrechs has finally done the honourable thing and resigned.
"The Tory leadership got him to fly back from Spain to vote for their budget.
"Without him, they would not have had a majority on the day.
"But it was an insult to local democracy and the people of Silverhill, who he was supposed to be representing.
"I don't think he felt comfortable with being asked to behave in this way and I am pleased he has brought this farce to an end."
Hastings Conservatives spokesman Terri Lock said Tory leader Peter Pragnell had accepted Mr Lambrechs' resignation 'with regret'.
Currently there are 15 Conservative, 10 Labour and five Lib Dem councillors.