The campaign kicks off in-store from 22 June with ads, product and environmental information, and recipes (including on M&S's own website), while external activity will include print and online advertising.
The initiative is being funded using the profits from the retailer's 5p carrier bag charge. Accordingly, its launch coincides with the rollout of a new bag design and Forever Fish branding in food halls and on its ready meals, Food To Go and frozen products.
The retailer has promised to give £1m to charity WWF over three years – money that will be used to better manage fish stocks and protect important species.
M&S has outlined a number of commitments: to set up and run School of Fish, an education programme targeting 400,000 primary school children; to encourage its 21 million customers and 78,000 staff to assist the Marine Conservation Society to clean 400-plus beaches twice a year, and to promote sustainably sourced fish species, such as dab and flounder, to customers.
Marc Bolland, chief executive of M&S, said: "We will work together with our customers, our people and their children, to promote a healthy future for our beaches, seas and fish.
"Forever Fish involves schools, charities, fishermen and fisheries, so that we can all enjoy cleaner beaches, more sustainable fishing and healthy fish."
Paul Willgoss, M&S's head of food technology, said: "Already 90% of the wild fish we sell is in line with our Plan A commitment, but certifying fish stocks is not the full story.
"We are working with our customers in support of the UK fishing industry, so that the next generation understands the importance of healthy oceans and can enjoy sustainably sourced fish."
Separately, M&S announced last week that it had poached Arcadia marketer Alison Jones as brand director for general merchandise, a new role created by chief executive Marc Bolland.
The week before, it reported an 11% lift in pre-tax profits for its full trading year after a concerted marketing drive helped lift sales in food and clothing.