The grocer has introduced a colour-code system - for now only available internally - that rates fish as green, amber or red, depending on the level of threat. It has vowed to only stock green- or amber-rated fish by the end of December.
If a fish is rated red, such as squid or swordfish, the supermarket will try to move it to amber by changing where it is sourced. If it cannot achieve this, it will no longer stock the fish.
Since February, Sainsbury's has been gradually removing endangered fish from its shelves - a change that has cost£1.5 million in lost sales.
The retailer will not be introducing the colour-code system in stores immediately, due to fears it may cause customer confusion with the traffic light system it uses to denote the nutritional value of fish.
However, it does plan to train 1,000 members of staff on the supermarket's fish counters next month, so they can help educate customers on endangered fish stocks.