Police said one of three suspects, all under the age of 15 according to Finnish broadcaster YLE, had admitted setting fire to an item that sparked the blaze which razed the wooden stand and burned part of the artificial pitch.
The incident came after the club, one of the country’s most decorated sides with nine league titles, were relegated from the Veikkausliiga, Finland’s top division, in October.
The club from the town of Valkeakoski, 150km north of Helsinki, has launched a fund-raising campaign to help it recover from the fire.
No wins in their final 15 Veikkausliiga matches saw Haka plummet to the foot of the table. A win on the final day at home to relegation rivals KTP would have saved them, but a 2-1 defeat sealed their drop to the Ykkosliiga for the 2026 season.