It has also introduced a promotional all-in fare on the route, starting at just Bt3,990 per flight.
The added flight to the Japanese city commences on December 1, chief executive officer Nadda Buranasiri said on Monday.
The promotional fare is available until September 24 for travel between September 18 and February 28.
Thai AirAsia X is a low-fare long-haul carrier that operates Airbus A330 aircraft featuring a total of 377 seats, comprising 12 premium flatbeds and 365 economy-class seats.
It currently operates from Bangkok (Don Mueang Airport) to Seoul three times a day, Tokyo twice a day, Shanghai once a day, and Osaka once per day.