John Leahy has resigned as leader of Renua Ireland.
The move by the Offaly councillor leaves the party without any elected representative.
He will reportedly stay on as an independent councillor.
He was elected in June 2009 and became leader of the party in September 2016, succeeding Lucida Creighton.
Renua, which describes itself as the Christian democratic party of Ireland, was founded four years ago by Lucinda Creighton and Eddie Hobbs.
In the general election in May 2016, all its three sitting TDs lost their seats.
Speaking in December that year, Mr Leahy told Newstalk: "Time was utterly against us".
He claimed there was a "swing against some of our policies" and said it "suit[ed] political parties to call us right wing... and that stuck for some odd reason."
Mr Leahy also denied Ms Creighton was a liability to Renua because of her stance on abortion.
As a junior minister, Ms Creighton broke with Fine Gael when it proposed legislation to address the 'X Case'.
"She was very brave... she stood up for her principles; she never inflicted that on the party," he said.