Britain's media regulator fined Playboy 100,000 pounds ($160,000, 121,000 euros) on Wednesday for its "reckless" failure to prevent children from accessing pornography.
Two websites owned by the US adult brand provided access to "hardcore videos and images" without adequate controls ensuring that users were aged 18 or over, the Ofcom regulator said.
"Playboy's failure to protect children from potentially accessing these sites was serious, repeated and reckless," the watchdog said.
The Demand Adult website displayed hardcore pornography that could be viewed simply by clicking on a button labelled "Enter, I am over 18" while the Playboy TV site used a similar age verification system to show less explicit content.
Users of both websites could access hardcore material using a debit card -- which unlike a credit card, can be obtained by Britons from the age of 11.
Ofcom said neither the button marked "I am over 18" nor the debit card payment system constituted "an effective age verification system".
Playboy has grown from the men's magazine founded by Hugh Hefner in 1953 into a global business empire spanning pornography, members' clubs and merchandise branded with its iconic "bunny" logo.