All Japan locations of Mos Burger started selling a vegan and vegetarian-friendly, plant-based hamburger on May 21, 2020. Known as the Green Burger and featuring a soy patty, this menu item makes dining out, getting takeout, and ordering food delivery in Japan a lot simpler for those on plant-based diets, and offers a new choice to anyone curious. This makes it one of the very few chain restaurants and the first in fast food in Japan to offer a vegan option (other casual chains, like Coco Ichibanya and Royal Host, have vegan-friendly choices).
I went to try the burger for myself on the 21st at my nearest Mos Burger. Below are some of my thoughts on this vegan-friendly fast food option.

I took my Green Burger set to-go. My local Mos Burger had very clear signs about the new hamburger and it being plant-based, and it is an option on the regular menu (meaning it is likely not for a limited time only, as some items tend to be!). The Green Burger costs 538 yen without tax, making it around 100 to 200 yen more expensive than other burgers on the menu. Ordering it with a side of fries and a drink brought me to a 1,020 yen total.

This burger is tasty, with tomato, lettuce, and a chunky tomato/vegetable sauce. Mos Burger is known for its quality; all the vegetables were crisp and fresh, and the soy patty was well cooked. The buns are green, too, adding a cute touch.
However, it is small. According to the official website, it contains just 299 calories. Be sure to get sides–and a small set may not be enough. Overall, though, the burger has satisfying flavors and is worth ordering at least once.
The Green Burger is available at all Mos Burger locations in Japan. As of March 2020, it was sold at certain restaurants in Tokyo and Kanagawa Prefecture, but it is now on the menu of all branches in Japan. It has also been confirmed on Mos Burger UberEats menus!
Please be aware that due to the effects of COVID-19, Mos Burger locations may be closed or have changed their operating hours. Check directly with the restaurant you are interested in before visiting.
Website (Japanese): https://www.mos.jp/