Dumfries-based Scott Trawlers employs 50 people and runs five vessels responsible for around 15% of UK scallop landings.
It also has three refrigerated lorries and an engineering arm focused on products for scallop fishermen.
The acquisition, backed by Royal Bank of Scotland, is the first time Macduff has purchased an entire fleet of fishing vessels.
Andy Scott, of Scott Trawlers, said: "The two businesses are a great fit and that was very important to me as I looked for ways in which I could still operate and grow the business but in partnership with a like-minded operation.
"We have always been at the forefront of the scallop sector and this deal enables us to bring forward innovation from the catch through to the end product."
Euan Beaton, chairman of Macduff, has ambitious growth plans.
He said: "This significant deal enables us to expand our scallop business, allows us to have a full chain of custody from the sea to the customer, and consolidates a solid 15-year relationship between the two businesses.
"This is a really exciting time in Macduff Shellfish's continued expansion. Over the last 10 years we've quadrupled the business.
"In the next five years I'd like us to double it again through acquisition deals such as this and through organic growth, including diversifying into other shellfish categories."
Macduff, which has its headquarters in Mintlaw, Aberdeenshire, reported a £38m turnover in the 12 months to March 31, 2011.