Fish and chip competitor Deep Blue Restaurants will take over Harry Ramsden's 34 sites, as well as its brand, in a debt free deal.
The sale comes after a period of slow decline for the Yorkshire business, as the casual dining sector continues to battle rising costs and falling footfall.
James Low, founder and chief executive officer of Deep Blue, said: "We are delighted with this transaction which has accelerated our growth, making us the largest player in the market by a considerable margin.
"Our immediate focus will be on the integration of the business into the Deep Blue organisation.
"Our amalgamation with Harry Ramsden's is a key milestone in our development but we remain committed to exploiting the unparalleled opportunities within the sector for further expansion."
Tom Crowley, chief executive officer of Boparan Restaurant Group, said: "This is an important strategic move for BRG and one which enables us to simplify our future business and allow us to focus on the opportunities for our core brands of Giraffe, Ed's Easy Diner, Fishworks and the roll out of Slim Chickens, whilst still retaining an ongoing interest in the fish and chip sector."
Harry Ramsden's was started 90 years ago in West Yorkshire but has struggled recently, after falling to a £5 million loss in its most recently filed accounts and closing a number of restaurant sites.
Boparan Restaurant Group said it will retain an interest in the enlarged business as part of the move.
The move will also see Boparan Restaurant Group, which owns other chains such as Giraffe and Ed's Easy Diner, exit the business after more than nine years.
Earlier this year, the restaurant arm of Ranjit Boparan's business empire, announced the closure of 27 of its 70 Giraffe and Ed's sites as part of a major restructuring.
Boparan has also sold other parts of its overall business in recent years, such as Goodfella's frozen pizzas and the red meat division of Two Sisters, in a bid to improve its finances.
The original Harry Ramsden’s chippy, in Guiseley, was brought back to its former glory through a £500,000 refurbishment and turned into the flagship Wetherby Whaler restaurant in 2012.