The agreement calls for the creation of a company to develop the EUR3.5- billion pipeline between the Romanian port of Constanta and Trieste in Italy.
"It's a first step, a necessary one to bring this project to fruition," Romania's state secretary for economic affairs Viorel Palasca told a press conference.
The new company, made up of Romania's Conpet and Oil Terminal, Serbia's Transnafta and Croatia's Janaf, will be officially registered in London, where it will be based, within 45 days, he added.
Slovenia and Italy, which initially expressed interest in the pipeline, haven't yet named the companies representing them in the project, Palasca said.
"Italy is waiting for the creation of a new government but even if it does not take part, there are alternative routes (for the pipeline)," he added.
Vladimir Vrankovic from Janaf noted however "without Slovenia, there are still options, but without Italy, there isn't a project."
The project, which has the European Union's backing, calls for the construction of a 1,300-kilometer pipeline to bring oil from the Black Sea to Central Europe in a bid to reduce dependence on Russian oil.