The communications authority said Playboy exposed children to potentially harmful pornographic material.
The websites owned by Playboy, Playboy TV and Demand Adult, allowed users to access "hardcore videos and images" without having proper controls in place to make sure users were 18 and over.
The two websites are regulated by Ofcom and the Authority for Video On Demand (ATVOD) because they provide video access in a similar way to adult services broadcast on television and fall within UK jurisdiction.
Ofcom says there are numerous controls websites can use to verify age which includes asking for credit card details before adult content is made accessible. Credit cards are not available to under 18s.
The UK watchdog also pointed out that the homepage of Demand Adult displayed "hardcore pornographic material" that could be viewed by simply clicking a button labelled "Enter I am over 18." If users wanted to access additional paid for content, they were then asked for debit card details.
Ofcom called Playboy's failure to protect children from accessing videos "serious, repeated and reckless."
Demand Adult incurred a fine of £65,000 and Playboy TV received a fine of £35,000.
In December 2012, Ofcom also fined the adult content website, Strictly Broadband, £60,000 for failing to have effective controls in place to verify if users on the site were 18 and over.