The four-strong range, which is launched next month, can be cooked from frozen in the oven or microwaved in four minutes unlike Birds Eye's oven-cook only Simply Naked & Natural and Simply Bake to Perfection ranges.
Young's, which debuted microwaveable fishfingers earlier this month, has targeted first-year sales of £6m for the range, backed by a £2m marketing spend.
"Microwave cooking has got left behind in recent years, but we want to help consumers overcome the fiddly aspects they associate with cooking fish," said a spokeswoman. "Research tells us consumers would love to eat more fish, but many lack the confidence to cook and handle it."
Young's Cook in the Bag (rsp: £2.99 for a two-fillet 250g pack) is available in four variants: wild pink salmon, haddock, smoked haddock and cod. Each comes with its own flavoured butter inside the bag. Further fish species could be launched in future, depending on the success of the initial lines, added the spokeswoman.
The launch will be supported by in-store and online marketing, followed by a TV campaign in January. The packaging will also feature the Fish for Life label to show it has been sourced responsibly.
Birds Eye has been heavily investing in the fish-in-bag format over the past year, launching Simply Bake to Perfection and Simply Naked & Natural in March, as well a range of vegetable steam bags under the new Field Fresh brand this month.