Online sports betting is costing millions of gamblers big time — with one in nine bettors spending half of their quarterly retirement savings on gambling, according to a shocking new study.

Researchers found that around 11% of online sports bettors are problem gamblers — and they are more likely to be from lower income households, according to a study of financial transaction data from 184,000 homes.

More than one-quarter of Americans say they have an account with an online sports book such as DraftKings or FanDuel — meaning the pool of gamblers who bet too much is massive.

“I think we will look back and shake our heads a bit and wonder at how we were willing to just open the floodgates,” said study co-author Jason Kotter of the Marriott School of Business at Brigham Young University.

Researchers found that problem gamblers spend 5% or more of their monthly income on sports betting — and that’s on top of existing spending on entertainment or other forms of gambling such as lotto tickets.

Even more troubling, the data found an apparent negative correlation between money being spent on sports betting and dough going into 401K or other investment accounts — indicating problem gamblers appear to be treating betting as an alternative to saving for the future.

“After legalization, frequent bettors cut their deposits to brokerage accounts by about $220 per quarter,” Kotter said.

“Before legalization, the average quarterly investment was about $400, so this represents a roughly 50% cut in their brokerage investments,” he said.

Since the US Supreme Court struck down a federal ban on sports gambling in 2018, the betting market in America has exploded, with wagers increasing 190%, from $57.6 billion to $167 billion, between 2021 and 2025, according to figures from Track360.

Online and mobile betting are now legal in 30 states plus Washington, DC, while a further nine states allow sports gambling in physical locations such as casinos and tribal gaming venues.

Proponents say sports betting brings in crucial tax revenue, with states collecting $3.7 billion in 2025 alone.

But Kotter’s research found that, while most bettors have their gambling habits under control, a small portion are jeopardizing their futures and those of their families.

“When states legalized betting, people started betting a lot. That’s not at all a surprise. But what is a little bit more surprising is what the distribution of those bets looks like,” Kotter told The Post.

“The vast majority of betters bet very modestly,” he said. “But there’s this group of bettors who bet an awful lot. In fact, the bettors who bet most frequently end up depositing something like 5% to 5.5% of their income to sports betting apps.”

Most worryingly, it appears that problem sports bettors are concentrated among lower-income households.

“The households that are putting the very largest fractions of their income into betting apps are the lowest income households,” Kotter said.

While Kotter’s research only looked at sports betting, he warned that sites such as Kalshi, which allow people to trade directly on the outcomes of everyday world events, risk blurring the line between gambling and investing.

While his research focused on financial data, Kotter noted that other studies have identified the broader social harms caused by gambling.

“There’s other work out there by academics showing an increase in bankruptcies, an increase in credit card delinquencies, but then you also see non-financial harm,” he said.

“So you see there’s some work showing an increase in things like domestic partner violence and child maltreatment,” he said.

His views appear to have backing from an increasing number of Americans. About 43% of US adults said legal sports betting is a bad thing for society in a 2025 Pew Research Center survey, up from 34% in 2022.

As a result, lawmakers are weighing up tighter guardrails, including advertising bans, prohibiting college player prop bets to reduce max fixing risk, banning credit cards and establishing federal standards to replace the patchwork of state laws.