It said that "gambling firms must now stop unfair online promotions that trap players' money". The changes will apply to all promotions.
After a CMA investigation into whether the sector was breaking consumer protection law, three companies - Ladbrokes, William Hill, and PT Entertainment - will make changes to bonus promotions for online games such as roulette and poker.
The Gambling Commission, which worked with the CMA on its investigation, said companies across the whole sector must "promptly adopt similar changes". Guidance for gambling firms will be published later this year.
"Firms not doing so will face regulatory action from the Gambling Commission," the CMA said in its statement.
Gambling Commission executive director, Sarah Gardner, said: "Gambling firms must treat their customers fairly and not attach unreasonable terms and conditions to their promotions and offers."
Source: GMB / BBC