A significant employer on the eponymous islands, Shetland Catch said Norway Pelagic provided the funding to increase its shareholding in the business from 25% to 50%.
The deal values Shetland Catch at £12m.
Shetland Catch's chairman, John Goodlad, said the investment reflects a wider confidence in the industry and recognises the potential the company has to develop its business.
The company claims to operate the largest pelagic factory of its kind in Europe, which it says can guarantee customers worldwide the freshest frozen mackerel and herring on the market. It earns much of its revenue in export markets such as Russia.
Tor Vikenes, chief executive of Norway Pelagic, said: "We have increased our stake in Shetland Catch because of its excellent strategic location, especially in relation to the attractive mackerel and North Sea herring fisheries. Shetland Catch has an experienced management team and we look forward to working with our partners."
Norway Pelagic made its initial investment of £1.533 million in Shetland Catch in 2007.
In July, Shetland Catch had a £1.5m confiscation order and a £150,000 fine imposed on it at the High Court in Edinburgh after previously pleading guilty to assisting vessel skippers in making undeclared landings.
Between 2002 and 2005 Shetland Catch helped vessel skippers who falsely declared the quantity of fish they landed as a means of evading the annual fishing quota allowed to each vessel. The skippers and another processing firm made illegal landings of herring and mackerel worth tens of millions of pounds.
The latest filed accounts for Shetland Catch show the company increased pre-tax profit to £5.6m in the year to 31 March 2011, from £2.2m in the preceding year.
It provides work for around 100 people including seasonal staff.
Other shareholders listed in Shetland Catch's latest annual return include Lerwick Harbour Trust.
Shetland Catch managing director, Simon Leiper, said: "While we will continue to operate independently, we very much look forward to working with our partners in Norway and planning for the future with confidence."