The two companies will sell foods using Marine Stewardship Council-certified fish, which is caught sustainably without threatening further dwindling stocks.
Mark Johnson, of manufacturer Mars Petcare, told the Guardian people were increasingly aware of the importance of sustainability.
“The End of the Line film [a documentary about overfishing] has had a big impact,” said Johnson, the company’s UK general manager.
“We are now the first pet company to make a commitment to sustainable fish, and we hope that will act as a catalyst for the whole industry.”
The move is in response to rising demand for sustainable fish. Co-operative recently banned threatened species from its own brand products while Marks & Spencer became the first high street chain to sign up to WWF’s new seafood charter.
The UK’s eight million pet owners buy 1.5 million tonnes of food a year and globally there are an estimated 750 million pets who consume 20 million tonnes annually.
The MSC-certified fish will be available in Whiskas and Sheba brands to all European consumers by Christmas.
Environmental groups welcomed the news. Jason Clay of WWF said: “There is no quick fix to this problem but when companies as influential as Mars take a leadership role, it is great news for the world’s oceans.”