Economics professor Roland Vaubel said the idea that an independent Scotland would not be a EU member has "no basis" in European treaties.
Scotland's membership of the EU has become one of the central arguments in the independence debate.
The Scottish Government previously argued a Yes vote would mean "automatic" membership of the EU.
This view was disputed by Herman van Rompuy, president of the EU Council, who said an independent Scotland would have to "reapply" for membership. European Commission president Jose Manuel Barroso said: "An independent state ... has to apply [for] European membership according to the rules".
As a result, SNP ministers argued that Scotland's EU membership would be negotiated from inside the organisation.
Now Vaubel, a member of the Advisory Council to the German Federal Ministry of Economics and Technology, has published a paper that is closer to the SNP's position on the issue.
In The Political Economy of Secession in the European Union, he argues: "If, say, Catalonia seceded from Spain or Scotland from the UK, both would remain members of the European Union. The seceding state and the rump state would have to negotiate an agreement on how they wished to share the rights and obligations of the predecessor state. If they did not meet their joint obligations, both could be expelled by the international organisation."
Vaubel also stated: "The legal position taken by Barroso - and van Rompuy has no basis in the European treaties. Nor is there a precedent in EU law. Nor has this question ever been settled in any UN agreement or Vienna Convention. There are merely practices, and they vary."
SNP MSP Clare Adamson said: "As this paper makes clear, there is no basis in the EU treaties for anything other than Scotland remaining a member of the EU and for people in Scotland remaining EU citizens with a vote for independence. The biggest risk to Scotland's EU membership is remaining part of a UK that is being dragged into a more right-wing isolationist and anti-European position."
A spokesman for the pro-Union Better Together campaign said: "Even the SNP Government now accept that a separate Scotland would have to apply to join the EU."