The cheese, from Daiya, is now available in nine select Los Angeles locations.
According to the chain, it 'acts as the perfect complement to Fatburger's wildly successful vegan-friendly Impossible Burger'.
"Our hometown Los Angeles Fatburger fans can now order a one-of-a-kind burger menu item - a 100 percent vegan-friendly cheeseburger," Andy Wiederhorn, CEO of FAT Brands, said in a statement.
"As a longstanding LA brand, our team at Fatburger loves to bring together old Los Angeles traditions and today's trends and values, and our new vegan offerings are a great example of that.
"We pride ourselves in being early adopters of innovative new menu items, and I anticipate the Daiya slice will do extremely well, just like our Impossible Burger."
Bestselling Impossible Burger
Back in early 2018, the chain announced that the plant-based Impossible Burger would become available at all its American locations, after becoming a 'best-selling' item in LA.
Fatburger became the first national fast-casual chain to carry the Impossible Burger at all domestic locations.
Speaking at the time, Fatburger CEO, Andy Wiederhorn, said: "I knew from the moment we debuted the Impossible Burger patty in Los Angeles that this was going to do well with our customers - it quickly became one of our best-selling items. There's no doubt our customers will always love 100 percent beef Fatburgers, but we're hoping to engage both old and new fans alike with a top-notch meat-free option."
Both Daiya and Impossible Foods have come into controversy with vegan consumers in recent times, with the latter branding its patty plant-based rather than vegan, after testing one of its ingredients - soy leghemoglobin aka 'heme' - on rats.
Speaking about the testing, Impossible Foods CEO Pat Brown, a vegan of more than 16 years, published a statement titled The Agonizing Dilemma of Animal Testing, saying the core of his company's mission is to 'eliminate exploitation of animals in the food system', as well as reduce the impact of animal agriculture on the environment.
Daiya was accused of 'betraying its ethics and core values' after it was acquired by Japanese pharmaceutical company Otsuka, which tests on animals, for a staggering $325.5 million in 2017. Some former plans pledged to boycott the brand as a result.