Gambling companies with license to operate in the Scandinavian country may only offer customer bonuses at the first game opportunity.
The Gaming Inspectorate has found Betway and Mandalorian to have violated this legislation by offering bonuses on repeated occasions.
It says these are serious offences, for which it has fined Mandalorian SEK9m ($938,925) and Betway SEK5m.
Spelinspektionen contacted companies under its jurisdiction on 19 February to make it clear they must comply with legislation relating to bonuses.
While the Gaming Inspectorate did report some compliance from licensees, it assessed that further measures were necessary.
Spelinspektionen says the legislation on one-time bonuses hopes to strengthen consumer protection and reduce the negative effects of gambling.
