Amazon is cracking down on the practice of companies offering free products to customers in exchange for a review.
In future, such reviews can only come via the online store's own program, Amazon Vine.
A recent study suggested that those who received free or discounted items were much more likely to write a favourable review.
Having product reviews that shoppers can trust has been a huge factor in Amazon's success.
The online retail giant has always prohibited compensation for reviews but had made an exception for reviewers that disclosed that fact.
"These so-called 'incentivised reviews' make up only a tiny fraction of the tens of millions of reviews on Amazon, and when done carefully, they can be helpful to customers by providing a foundation of reviews for new or less well-known products," Chee Chew, vice-president of customer experience at Amazon, wrote in a blog post.
But now it has decided to ban them completely - other than through Amazon's own program, Vine.
"Creating, modifying, or posting content in exchange for compensation of any kind (including free or discounted products) is not allowed," read the updated community guidelines.
A recent study of more than seven million reviews by analysis site ReviewMeta found that the average rating for products with incentivised reviews was higher than those for non-incentivised ones.
Amazon's Vine program invites trusted reviewers to post opinions about new and pre-released items to help shoppers make informed purchase decisions.
"Vine has important controls in place and has proven to be especially valuable for getting early reviews on new products that have not yet been able to generate enough sales to have significant numbers of organic reviews," said Mr Chew.
Reviewers are chosen by Amazon and tend to be people who have developed a reputation for expertise in specific products.
Last year, Amazon sued four companies that it said paid people to produce reviews.
It is also taking legal action against more than 1,000 people it says have offered to submit positive reviews to the site in return for payment.