The two sites, which the regulator didn't identify, are YouPorn and RedTube, local media reported. The sites allow users to post and download sex videos.
"It should be noted that the hardcore pornographic videos posted on these sites are very easily accessible by the young, as each video will start streaming for free once a user clicks on the related link," said Jason Hoong, an official from the Media Development Authority.
Authorities had been criticized over the two sites, which have been added to Singapore's "mass impact objectionable" list of 100 banned pornographic websites.
Singapore forbids the possession, distribution and production of pornographic films and justified its decision based on a desire to protect the youth from accessing adult websites.
A local online report on the ban drew negative responses from those who said the government's decision was unrealistically moralistic.
"I will definitely surf the Internet by proxy from now on, to be defiant so as to preserve my rights," one commenter wrote.