Betting firms Ladbrokes, William Hill, and PT Entertainment have agreed to change online games promotions after pressure from the regulator.
The Competition and Markets Authority (CMA) said punters must be able to cash out when they want, and not have to play more to release winnings.
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.
- Players will not be required to play multiple times before they can withdraw their own money
- Gambling firms must ensure that any restrictions on gameplay are made clear to players, and cannot rely on vague terms to take players' money
- Gambling firms must not make players take part in publicity to collect winnings
The Gambling Commission, which worked with the CMA on its investigation, said companies across the whole sector must "promptly adopt similar changes".
"Firms not doing so will face regulatory action from the Gambling Commission," the CMA said in its statement.
Guidance for gambling firms will be published later this year.
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."
Brian Wright, director of business for the Remote Gambling Association said: "There are clearly lessons to be learned for some companies and we will work with the Gambling Commission and others to raise standards wherever necessary.
"We have already held productive discussions with the CMA and the Commission to consider how best to achieve that."
A Ladbrokes Coral statement said the company recognised that "things had unintentionally gone too far" and that the new rules would improve transparency.
"It is never comfortable being in the spotlight on these sorts of issues but we are pleased that a way forward has now been identified and are committed to ensuring we meet the standards set," the company said.
William Hill said in a statement: "As one of the largest online betting and gaming brands in the UK, William Hill has worked with the CMA to ensure that its concerns have been fully met.
"We welcome the standards and principles that the CMA has outlined and we look forward to their adoption across the industry."