The Impossible Whopper is only available at the St. Louis outlet for a limited time and one fan went to get it as soon as he heard the news.
"I bought a second for a diehard meat eater that straight said he would not have known that it was plant-based," he said.
"I kept looking at mine trying to make sure it WAS plant-based. This is amazing seeing plant-based options in fast food."
The Impossible Whopper has mayonnaise, which can be omitted by request, and is cooked on a surface shared with conventional meat.
If successful, it may be rolled out across the chain's more than 7000 locations in the US later this year.
Pat Brown, chief executive of Impossible Foods, confirmed the news, saying people should think of it as "a 'meta' April Fool's Joke".
"People will get a burger that they will actually believe it's made from an animal, and be told it's made from plants, and think it's an April Fool's joke and it's not!"
The company's aim is to completely replace animals in the food system and it decided to tackle beef first because they claim beef production is the most environmentally destructive part of the animal agriculture system.
However, in Australia, Hungry Jack's' foray into the vegan market hasn't gone done as well.
A road-testing of the chain's vegan "cheeseburger" which debuted in more than 400 restaurants across the country in October a permanent menu item got labelled the "blandest thing I have ever eaten".
"You know a burger is in trouble when the dominant taste is the shredded iceberg lettuce," the reviewer wrote.
Burger King currently features a vegetarian burger on its menu and several other items are vegan, including French Toast Sticks, Dutch Apple Pie, Garden Side Salad, Oatmeal, Hash Browns and French Fries.
