This is the regulator’s second fine imposed under the law – which requires pornographic websites to verify users are over 18.
Its first fine was to online message board 4chan, which it said had not responded to requests for information about measures required to prevent people from accessing illegal content.
Ofcom had set a deadline of 13 November to pay the £20,000 fine.
But a lawyer for the US-based company previously told the BBC it had no intention of paying, accusing the regulator of overreach and censorship.
If companies do not pay up, the regulator can seek to recover fines through legal action, or apply for court orders to instruct internet providers to limit access to the site.
Since 25 July, sites hosting age-restricted content have to have “highly effective age assurance” methods in place to make sure children cannot access them.
These include age verification tools such as facial age estimation, which must be “robust” in deducing whether a user is over 18.
Pornhub, the world’s most visited porn site, told the BBC in October it had seen a 77% decline in UK visitors since it implemented age checks.
Ofcom has also opened new investigations into 20 further porn sites – bringing the number of sites and apps it is investigating for suspected Online Safety Act breaches to 76.
Meanwhile, recent reports, external suggest the government may look to ban so-called “nudifying” or “declothing” apps as part of further efforts to tackle deepfake intimate image abuse.