More than 40 firefighters tackled the blaze that caused extensive damage at Leyland Warriors in Moss Side Way, Lancashire, at about 02:00 BST.
The repairs will take months, the club says, affecting more than 200 players.
Lancashire Fire and Rescue Service said no-one was hurt, but it is believed the fire was "started deliberately".
Head coach Phil Roberts said everybody was "in a state of shock" after the fire.
Leyland Warriors play in the second division of the North West Men's League.
Volunteers at an amateur rugby league club say they are "heartbroken" the clubhouse has been badly damaged in a suspected arson attack.