It will be called RISE, which stands for Respect, Independence, Socialism and Environmentalism, and plans to have candidates standing in the 2016 Holyrood election.
Based on a coalition of the Scottish Left Project and the Scottish Socialist Party (SSP), it describes itself as a grass-roots movement which grew out of the independence referendum.
Organisers of today’s launch in Glasgow said hundreds of people were gathering from across Scotland to take part in workshops and listen to a range of speakers addressing the event.
Former MSP Colin Fox, national co-spokesman of the SSP, told those gathered at the Marriott Hotel: “This is the most important left unity project Scotland has seen in a generation.
“Since we last met here in this very hotel room as the Radical Independence Campaign, Scotland has changed beyond all recognition.
“We here, who were so involved in the Yes movement, should be immensely proud of what we achieved and what we changed, even though ultimately the vote went against us.
“Yet for all that has changed politically, far, far, far too little has changed socially and economically as far as the improvements secured by working-class people in Scotland are concerned.”
Mr Fox said RISE was an “important ingredient” in calling for Scottish independence.
He added: “We want to make it absolutely clear that the independence movement does not belong to the SNP, it is not Nicola’s plaything.
“We reiterate here today that supporting Scotland’s democratic right to self-determination does not make you a Scottish nationalist; it makes you a democrat and that’s the power behind the idea.”
Mr Fox said the movement had its “sights set” on becoming the “left opposition to the SNP” and to win seats in the Scottish Parliament next year where it will field candidates on regional lists.
“We have a tremendous opportunity, using the platform that Holyrood provides, to raise socialist ideas inside the parliament and outside after a nine-year absence.
“Our message here this morning is that the place for socialists in Scotland is in RISE - Scotland’s left alliance. We intend to challenge those who seek to represent working-class people in Scotland.”