The website, riksdagen.se, was taken down at 11am on Tuesday, with visitors met by a blank screen.
By 2pm, the website was up and running again, but officials confirmed that the problem had not been caused by any internal IT troubles.
"It went down because of an attack from the outside," Riksdag spokesperson Anna Olderius told the TT news agency.
"But we refuse to comment on security issues in any more detail than that."
The cyber attack marks the second against the website in the past two years. In October 2012, the website went down together with that of the country's central bank other government websites, news networks, and university home pages.
Hacktivist network Anonymous claimed responsibility for the October attacks.
"You don't fuck with the internet," the group wrote online, in what was apparently a response to police raids on the previous hosts to The Pirate Bay and WikiLeaks.
The attacks were carried out via a Distributed Denial of Service (DDOS), where a website is bombarded with communication requests so that the servers become overloaded and the site crashes.
As yet, no one has claimed responsibility for Tuesday's attack.