Dublin South West TD Paul Murphy has left the Socialist Party.
Mr Murphy is expected to launch a new socialist group next week.
He has been one of the most prominent socialist TDs in the Dáil since he was first elected in 2014.
In a statement Mr Murphy said: "After an extensive debate within the Socialist Party, a group of members, including me, have decided to leave and intend to launch a new socialist group.
"When we decided to leave, the party accepted that and we all aim to have an amicable and cooperative relationship in the future in the interests of working class people."
Since the General Election of 2016 Deputy Murphy has been part of the Solidarity-PBP group which has six TDs.
He will remain part of this umbrella group in the Dáil chamber, but he has broken away from his two Solidarity colleagues Ruth Coppinger and Mick Barry.
It is another split on the left of Irish politics despite repeated efforts there to create a greater sense of unity.
It follows a difference of opinion on whether or not the Socialist Party should get involved in broader movements with other parties such as Sinn Féin and the Green Party.
The TD, who became a well known figure during the anti-water charges protests, advocated more engagement with politicians and activists outside the ranks of socialist politics.