The sale by Mr Sadleir, who quit Scotgold's board in April, comes as the company continues to seek financing to open the Cononish mine after recent falls in the gold price.
Scotgold yesterday told investors that, at the end of last week, it was notified by Mr Sadleir he had sold 3,937,500 shares.
The sale price was not disclosed but at prevailing prices the shares would have been worth £40,000.
Mr Sadleir is a soil scientist and geologist who has been involved in exploration projects in Australia.
He retains around six million shares, worth £66,000.
His holding has fallen from 4.7% to below 3% mark at which he must notify Scotgold and the stock market about changes in his stake.
The latest sale comes two months after Scotgold revealed that Mr Sadleir had sold more than five million shares in batches between April 17 and May 27.
Scotgold, led by Chris Sangster, needs to raise around £23.2 million to develop the mine near Tyndrum. Financing from its South African-based banker RMB Resources is conditional on it raising sufficient equity for the project.
It put its plans on hold in May and told investors it was halting discretionary expenditure.
Scotgold, which is listed on the Alternative Investment Market and in Australia, received planning permission in 2012 to develop the site. It expects to recover 121,800 ounces of gold and 469,700 ounces of silver.
In its final development study published in April, Scotgold said at prevailing prices it expected £39.8m of pre-tax cashflow over the mine's seven-year life.
This would allow it to pay back development costs 26 months after the start of production.
A year ago when gold was trading at $1655 an ounce, it predicted cash of £65.9m.