Graham and Christine Donaldson have established a 3.4% holding in the company, according to a filing with the stock exchange.
"He is a real enthusiast," Scotgold chairman John Bentley said of Mr Donaldson, who is now understood to be based in England.
Mr Bentley said he had met the private investor "a couple of times".
Mr Bentley said that Mr Donaldson has been building his holding in the company, which is listed on the Australian Stock Exchange as well as the junior Alternative Investment Market in the UK, over the last couple of years.
He added: "There is nothing really to read into it other than enthusiasm."
With 3.43% of Scotgold, the Donaldsons are its largest investors ahead of chief executive Chris Sangster with 3.04%, according to Reuters data.
The stock building by the Donaldsons comes at a tricky time for Scotgold.
In November it announced it is significantly scaling back its planned gold mine at Cononish to focus initially on extracting higher grade deposits after a fall in the price of the precious metal left it struggling to raise development finance.
Last month the company secured A$743,000 (£406,000) from investors to obtain a loan extension from its banker and give it time to develop its plans.
Scotgold has indicated it is now likely to seek £10 million in capital to start production at the mine near Tyndrum. This is down from the £25m originally proposed.