Cancer Research UK’s survey found 86% thought offering extra items for free encouraged people to buy junk food.
Jaimie Treharne, from Ammanford, said so-called ‘bogof’ offers were one of the main reasons she put on weight.
The Welsh Government said it was “considering a range of proposals” to help people stay a healthy weight.
Retailers say they are “fully committed to the health of their customers” and “playing a leading role in offering and promoting healthy options across their outlets”.
The survey of 1,067 adults in Wales also found 68% of those polled thought temporary price reductions influenced them to buy unhealthy food.
The results were published ahead of the Welsh Government launching a three-month consultation into how it should put in place its Healthy Weight, Healthy Wales strategy.
According to the charity, 13 types of cancer are linked to obesity and about 1,000 cases of cancer a year are down to being overweight or obese.
Obesity in Wales is worse than any other UK nation – 59% of adults are overweight, with 23% classed as obese, according to the National Survey for Wales 2016-17.
Mother-of-two Jaimie Treharne, 31, dropped from a size 24 to a size 14/16 after changing her eating habits.
She said: “Buy-one-get-one-free offers were one of the main reasons I put on weight. I would buy biscuits and crisps that were on offer thinking they would last a few weeks.
“That never happened, and I would binge eat as the temptation was too strong.
“I believe limiting the multi-buy offers on food that’s high in fat will encourage families to make better choices and ultimately live healthier lives.”
Ms Treharne witnessed her mother undergoing cancer treatment and, while it was not weight-related, she said she was determined to live life more healthily to stack the odds of not getting the disease in her favour.
“I want to do everything I can to see my children grow up and I know eating healthily will help me achieve this.”
Ms Treharne, who is now a weight plan consultant, said: “I can’t look at my wedding pictures as that’s when I was at my biggest. Something clicked one day and I decided I couldn’t go on living the way I had been.
“Now, my mindset has completely shifted and I now go out of my way to avoid the big supermarkets with multibuy offers so I can make healthier choices for my family.”
Cancer Research UK say the offers are “unhelpful and unhealthy” and “persuaded people to ignore their shopping lists and buy cheap junk food in large quantities”.
“By restricting special offers on unhealthy food and drink, the Welsh Government can do something really effective to influence the contents of our shopping baskets and help us all keep a healthier weight,” said Andy Glyde, from the charity.
The Welsh Retail Consortium said retailers were already “fully committed to the health of their customers and are playing a leading role in offering and promoting healthy options across their outlets”.
“Welsh customers benefit from a wide range of healthy, fresh and affordable options when shopping and retailers are leading the way in providing clear nutritional information,” said Sara Jones, the Welsh Retail Consortium’s head.
“All of which helps customers understand their food and make informed choices.”
The Welsh Government said it is “committed to reducing levels of obesity” and is “considering a range of proposals for our approach to supporting people to achieve and maintain a healthy weight”.
“Our approach is to promote a positive healthy lifestyle throughout life, with an early focus on preventing overweight and obesity in childhood,” said a spokesman.