The decision to sell the 25% stake to the company that owns the Masterpiece fair follows the sale by Stanley Gibbons earlier this month of auction house Dreweatts and dealer Mallett for £2.4m.
The group became the owner of part of Masterpiece when it acquired Mallett in 2014 for £8.6m. Mallett was one of the four founding firms of the London fair, which this year takes place between June 28- July 5.
The stock exchange-listed stamp dealer will use the proceeds of the sale to pay down debt.
It said for the year to March 31 a dividend of £40,000 was received from Masterpiece.
Stanley Gibbons will now concentrate on its stamps and coins businesses comprising Baldwin's, Stanley Gibbons, Murray Payne and Apex.
It continues to own rare book auction and retail business Bloomsbury and the inventory of Mallett, but said in a statement it will "derive additional value" from these assets in the coming months.