"Out Of Town Affairs" brings together married men and women seeking sex, the Daily Mail reported.
It has been clicked on 52,375 times in just seven months via computers exclusively used by members of the houses of parliament and their employees.
In one month, the website received 289 hits a day from parliament.
In December 2012, it got more hits from parliamentary computers than the official websites for the treasury, justice ministry and education department.
The website, which contains explicit pictures of subscribers, says: "We connect you with thousands of other married men and women looking for a marital affair or more heat in the bedroom."
"Marital affairs in Britain (are) very popular - with more and more unsatisfied married men and women looking to do the dirty (on) their partners whilst out of town," it says.
Users are invited to contact "thousands of horny cheaters ready to get some action" by signing up for a fee of 25 pounds a month.
The website also allows them to "write your own sex-date diary" and "read and send filthy messages".
The Daily Mail made the discovery after submitting a Freedom of Information request, asking for the top 500 websites accessed on parliament computers over the past year.
The owner of the website said he was surprised that MPs were visiting the site.
"That's a lot of hits. It's got to be more than one person going on the site from parliament. I hope we don't get hate mail from parliamentary wives now," he said.
Parliament officials are also spending time on online gambling, playing games and chatting on social network sites.
There are thousands of hits on gambling websites, while i-am-bored.com, featuring videos, games and social networking had 21,371 hits in the last three months of 2012.
Facebook clocked up 28 million hits over a year.
A parliament spokesman said more than 5,000 people had access to parliamentary network computers but that the "Out Of Town Affairs" website has now been blocked.