The Information Commissioner's Office said it issued the penalty worth 14.5 million pounds ($A27.8 million) because the failures resulted in the platform using children's data "unlawfully."
"Children under 13 had their personal information collected and used in ways they could not understand, consent to or control. That left them potentially exposed to content they should not have seen," said Information Commissioner John Edwards.
"This is unacceptable and has resulted in today's fine."
The UK privacy regulator has been escalating scrutiny of online platforms over child safety. Earlier this month it hit MediaLab, owner of image-sharing site Imgur, with a 247,590 pound ($A474,150) fine over similar failures and it has also been investigating TikTok since last year.
The watchdog took issue with Reddit's age verification measures. It said that even though the platform doesn't allow children under 13 to use its service, it didn't have any way to check the ages of its users before July 2025.
Edwards said online platforms that are likely to be accessed by children are responsible for protecting them by making sure they're not exposed to any risks "through the way their data is used".
They can do this with "effective age assurance measures," he said.
Reddit rolled out age verification measures in July 2025 in order for users to access mature content, including asking them to declare their age when setting up an account.
But the watchdog said "self-declaration" is easy to bypass and that it told Reddit it would continue to monitor the platform's handling of children's data.
Reddit said it would appeal the decision.
"Reddit doesn't require users to share information about their identities, regardless of age, because we are deeply committed to their privacy and safety," the company said in a statement.
"The ICO's insistence that we collect more private information on every UK user is counterintuitive and at odds with our strong belief in our users' online privacy and safety."
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