James Hamilton, a former director of public prosecutions in Ireland, is the latest appointment to the panel of independent advisers on the ministerial code.
He joins former Scottish lord advocates Lord Peter Fraser and Dame Elish Angiolini, who have served as independent advisers since August 2011. Sir David Bell, a former top civil servant, was appointed in October to investigate comments Alex Salmond made about legal advice on Scotland's future in Europe.
Mr Salmond said the independent advisers' advice has been "highly valued" since the panel was created in 2008. "I am delighted to welcome James Hamilton as a new independent adviser, ensuring the continuation of the robust procedures that we have within Scotland's system of government," he said.
Six complaints have been referred to the independent panel to date, all of which were dismissed.
The most recent row centred on comments Mr Salmond made when he appeared to suggest ministers had taken legal advice on an independent Scotland's status in the EU, when it emerged no such advice existed.