The added flights will coincide with cherry-blossom season in Japan, a popular draw for Thai tourists, who know the blooming trees as sakura.
Promotional fares will start at Bt3,333 one-way.
Chief executive Nadda Buranasiri said bookings could be made from January 29 to February 4 at www.airasia.com for travel between March 25 and October 27.
Tokyo is the No 1 destination for Thai travellers to Japan, Nadda said.
The city has high potential for repeat visits, group tours and “fully independent travellers” (FIT) because it’s a jumping-off point to the World Heritage City of Nikko and the natural splendour of Nagano, which are becoming increasingly popular.
Thai AirAsia X currently has direct flights from Don Mueang to Seoul in South Korea; Osaka, Sapporo and Tokyo in Japan; and Shanghai in China.
The Ministry of Tourism and Sports says 1.5 million Japanese visited Thailand in 2017, up from 1.4 million in 2016.