The retailer has acquired the former Kerry Foods site that has been empty since 2012 for an undisclosed amount.
Morrisons currently produces 250t of fish a week at its existing 35,000m2 site.
The new site will increase the space to cover more than 120,000m2 and include new packaging, chilling and filleting sections.
Mark Harrison, group md of Morrisons, said: “Not only will we be able to supply a wider product range but we’ll also be increasing the number of skilled jobs available in the local area.
“The new space will mean we can manage the entire process of preparing fish ourselves and because that’s efficient we’ll be able to offer even better value to our customers.”
In addition to the two sites in Grimsby, Morrisons also opened a facility in 2014 at the Humberside Seafood Institute to serve the supermarkets growing online and convenience services.
Morrisons was the only UK supermarket to have an extensive network of vertically-integrated food processing sites and abattoirs, it claimed.
Kerry Foods blamed increasing competition in the frozen meal sector and falling sales for the initial closure of the plant.
Production at the site was transferred to the firm’s Carrick Macross facility in County Monaghan, Ireland.
The closure caused controversy at the time as Kerry Foods refused to talk to Unite the union over the loss of 337 jobs.
The site was previously owned by UK-based ready meals firm Headland Foods, which was acquired by Kerry Foods in December 2010.
Following the deal, in April 2011, Kerry was also forced to close Headland’s plant in Flint, North Wales.
Meanwhile, last week, Morrisons boss Dalton Philips was sacked after five years of challenges, controversy and criticism from analysts and former bosses.
A fall in total sales – excluding fuel – by 1.3% for Morrisons in the six weeks to January 4 2015 was the final nail in the coffin for Philips, with his departure revealed along with the results.
To view our photogallery of Philips’s final year at the firm, click here.
Meanwhile, to view the very latest jobs in UK food and drink manufacturing, visit FoodManJobs.