This is the third title from the Ultraman franchise that received this award. Previous winners from the franchise include Ultraman Tiga and Ultraman Z.
The Seiun Award awards the best sci-fi works and achievements of the previous year. It is similar to another sci-fi award, the Hugo Award in which the attendees of each respective convention vote on the winners. The nominees were chosen among the titles that were released between January 1 to December 31, 2022. The voting for this year's Seuin Award ran from April 13th to June 31st. The award ceremony happened last August 5th at the 60th Japan Science Fiction Convention.
[via Anime News Network]